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Category Archives: Object Oriented PHP

Object-Oriented PHP for Beginners

Understanding Object-Oriented Programming

Object-oriented programming is a style of coding that allows developers to group similar tasks into classes. One of the major benefits of OOP is that, if a piece of information changes in your program, usually only one change is required to update the code.

OOP is intimidating to a lot of developers because it introduces new syntax and, at a glance, appears to be far more complex than simple procedural, or inline, code. However, upon closer inspection, OOP is actually a very straightforward and ultimately simpler approach to programming.

Understanding Objects and Classes

A class, for example, is like a blueprint for a house. It defines the shape of the house on paper, with relationships between the different parts of the house clearly defined and planned out, even though the house doesn’t exist.

An object, then, is like the actual house built according to that blueprint. The data stored in the object is like the wood, wires, and concrete that compose the house: without being assembled according to the blueprint, it’s just a pile of stuff. However, when it all comes together, it becomes an organized, useful house.

Classes form the structure of data and actions and use that information to build objects. More than one object can be built from the same class at the same time, each one independent of the others. Continuing with our construction analogy, it’s similar to the way an entire subdivision can be built from the same blueprint: 150 different houses that all look the same but have different
families and decorations inside.

Structuring Classes

The syntax to create a class is pretty straightforward: declare a class using the class keyword, followed by the name of the class and a set of curly braces ({}):

<?php

class MyClass
{
    // Class properties and methods go here
}

?>

After creating the class, a new class can be instantiated and stored in a variable using the new keyword:

$obj = new MyClass;

To see the contents of the class, use var_dump():

var_dump($obj);

Try out this process by putting all the preceding code in a new file called test.php in [your local] testing folder:

<?php

class MyClass
{
	// Class properties and methods go here
}

$obj = new MyClass;

var_dump($obj);

?>

Load the page in your browser at http://localhost/test.php and the following should display:

object(MyClass)#1 (0) { }

In its simplest form, you’ve just completed your first OOP script.

Defining Class Properties

To add data to a class, properties, or class-specific variables, are used. These work exactly like regular variables, except they’re bound to the object and therefore can only be accessed using the object.

To add a property to MyClass, add the following code to your script:

<?php

class MyClass
{
    public $prop1 = "I'm a class property!";
}

$obj = new MyClass;

var_dump($obj);

?>

The keyword public determines the visibility of the property, which you’ll learn about a little later in this chapter. Next, the property is named using standard variable syntax, and a value is assigned (though class properties do not need an initial value).
To read this property and output it to the browser, reference the object from which to read and the property to be read:

echo $obj->prop1;

Because multiple instances of a class can exist, if the individual object is not referenced, the script would be unable to determine which object to read from. The use of the arrow (->) is an OOP construct that accesses the contained properties and methods of a given object.
Modify the script in test.php to read out the property rather than dumping the whole class by modifying the code as shown:

<?php

class MyClass
{
    public $prop1 = "I'm a class property!";
}

$obj = new MyClass;

echo $obj->prop1; // Output the property

?>

Reloading your browser now outputs the following:

I'm a class property!

Defining Class Methods

Methods are class-specific functions. Individual actions that an object will be able to perform are defined within the class as methods.
For instance, to create methods that would set and get the value of the class property $prop1, add the following to your code:

<?php

class MyClass
{
    public $prop1 = "I'm a class property!";

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

$obj = new MyClass;

echo $obj->prop1;

?>

Note — OOP allows objects to reference themselves using $this. When working within a method, use $this in the same way you would use the object name outside the class.
To use these methods, call them just like regular functions, but first, reference the object they belong to. Read the property from MyClass, change its value, and read it out again by making the modifications below:

<?php

class MyClass
{
    public $prop1 = "I'm a class property!";

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

$obj = new MyClass;

echo $obj->getProperty(); // Get the property value

$obj->setProperty("I'm a new property value!"); // Set a new one

echo $obj->getProperty(); // Read it out again to show the change

?>

Reload your browser, and you’ll see the following:

I'm a class property!
I'm a new property value!
<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

// Create two objects
$obj = new MyClass;
$obj2 = new MyClass;

// Get the value of $prop1 from both objects
echo $obj->getProperty();
echo $obj2->getProperty();

// Set new values for both objects
$obj->setProperty("I'm a new property value!");
$obj2->setProperty("I belong to the second instance!");

// Output both objects' $prop1 value
echo $obj->getProperty();
echo $obj2->getProperty();
?>

When you load the results in your browser, they read as follows:

I'm a class property!
I'm a class property!
I'm a new property value!
I belong to the second instance!

As you can see, OOP keeps objects as separate entities, which makes for easy separation of different pieces of code into small, related bundles.

Magic Methods in OOP

To make the use of objects easier, PHP also provides a number of magic methods, or special methods that are called when certain common actions occur within objects. This allows developers to perform a number of useful tasks with relative ease.

Using Constructors and Destructors

When an object is instantiated, it’s often desirable to set a few things right off the bat. To handle this, PHP provides the magic method __construct(), which is called automatically whenever a new object is
created.

For the purpose of illustrating the concept of constructors, add a constructor to MyClass that will output a message whenever a new instance of the class is created:

<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

// Create a new object
$obj = new MyClass;

// Get the value of $prop1
echo $obj->getProperty();

// Output a message at the end of the file
echo "End of file.";
?>

Note — __CLASS__ returns the name of the class in which it is called; this is what is known as a magic constant.

Reloading the file in your browser will produce the following result:

The class "MyClass" was initiated!
I'm a class property!
End of file.

To call a function when the object is destroyed, the __destruct() magic method is available. This is useful for class cleanup (closing a database connection, for instance).
Output a message when the object is destroyed by defining the magic method __destruct() in MyClass:

<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

// Create a new object
$obj = new MyClass;

// Get the value of $prop1
echo $obj->getProperty();

// Output a message at the end of the file
echo "End of file.";
?>

With a destructor defined, reloading the test file results in the following output:

The class "MyClass" was initiated!
I'm a class property!
End of file.
The class "MyClass" was destroyed.

To explicitly trigger the destructor, you can destroy the object using the function unset():

<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

// Create a new object
$obj = new MyClass;

// Get the value of $prop1
echo $obj->getProperty();

// Destroy the object
unset($obj);

// Output a message at the end of the file
echo "End of file.";
?>

Now the result changes to the following when loaded in your browser:

The class "MyClass" was initiated!
I'm a class property!
The class "MyClass" was destroyed.
End of file.

Converting to a String

To avoid an error if a script attempts to output MyClass as a string, another magic method is used called __toString().

Without __toString(), attempting to output the object as a string results in a fatal error. Attempt to use echo to output the object without a magic method in place:

<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

// Create a new object
$obj = new MyClass;

// Output the object as a string
echo $obj;

// Destroy the object
unset($obj);

// Output a message at the end of the file
echo "End of file.";
?>

This results in the following:

The class "MyClass" was initiated!

Catchable fatal error: Object of class MyClass could not be converted to string in /Applications/XAMPP/xamppfiles/htdocs/testing/test.php on line 40

To avoid this error, add a __toString() method:

<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function __toString()
    {
        echo "Using the toString method: ";
        return $this->getProperty();
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

// Create a new object
$obj = new MyClass;

// Output the object as a string
echo $obj;

// Destroy the object
unset($obj);

// Output a message at the end of the file
echo "End of file.";
?>

In this case, attempting to convert the object to a string results in a call to the getProperty() method. Load the test script in your browser to see the result:

The class "MyClass" was initiated!
Using the toString method: I'm a class property!
The class "MyClass" was destroyed.
End of file.

Using Class Inheritance

Classes can inherit the methods and properties of another class using the extends keyword. For instance, to create a second class that extends MyClass and adds a method, you would add the following to your test file:

<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function __toString()
    {
        echo "Using the toString method: ";
        return $this->getProperty();
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

class MyOtherClass extends MyClass
{
    public function newMethod()
    {
        echo "From a new method in " . __CLASS__ . ".";
    }
}

// Create a new object
$newobj = new MyOtherClass;

// Output the object as a string
echo $newobj->newMethod();

// Use a method from the parent class
echo $newobj->getProperty();
?>

Upon reloading the test file in your browser, the following is output:

The class "MyClass" was initiated!
From a new method in MyOtherClass.
I'm a class property!
The class "MyClass" was destroyed.

Overwriting Inherited Properties and Methods

To change the behavior of an existing property or method in the new class, you can simply overwrite it by declaring it again in the new class:

<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function __toString()
    {
        echo "Using the toString method: ";
        return $this->getProperty();
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

class MyOtherClass extends MyClass
{
    public function __construct()
    {
        echo "A new constructor in " . __CLASS__ . ".";
    }

    public function newMethod()
    {
        echo "From a new method in " . __CLASS__ . ".";
    }
}

// Create a new object
$newobj = new MyOtherClass;

// Output the object as a string
echo $newobj->newMethod();

// Use a method from the parent class
echo $newobj->getProperty();
?>

This changes the output in the browser to:

A new constructor in MyOtherClass.
From a new method in MyOtherClass.
I'm a class property!
The class "MyClass" was destroyed.

Preserving Original Method Functionality While Overwriting Methods

To add new functionality to an inherited method while keeping the original method intact, use the parent keyword with the scope resolution operator (::):

<?php

class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function __toString()
    {
        echo "Using the toString method: ";
        return $this->getProperty();
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    public function getProperty()
    {
        return $this->prop1 . "";
    }
}

class MyOtherClass extends MyClass
{
    public function __construct()
    {
        parent::__construct(); // Call the parent class's constructor
        echo "A new constructor in " . __CLASS__ . ".";
    }

    public function newMethod()
    {
        echo "From a new method in " . __CLASS__ . ".";
    }
}

// Create a new object
$newobj = new MyOtherClass;

// Output the object as a string
echo $newobj->newMethod();

// Use a method from the parent class
echo $newobj->getProperty();
?>

This outputs the result of both the parent constructor and the new class’s constructor:

The class "MyClass" was initiated!
A new constructor in MyOtherClass.
From a new method in MyOtherClass.
I'm a class property!
The class "MyClass" was destroyed.

Assigning the Visibility of Properties and Methods

For added control over objects, methods and properties are assigned visibility. This controls how and from where properties and methods can be accessed. There are three visibility keywords: public, protected, and private. In addition to its visibility, a method or property can be declared as static, which allows them to be accessed without an instantiation of the class.

Public Properties and Methods

All the methods and properties you’ve used so far have been public. This means that they can be accessed anywhere, both within the class and externally.

Protected Properties and Methods

When a property or method is declared protected, it can only be accessed within the class itself or in descendant classes (classes that extend the class containing the protected method).

Declare the getProperty() method as protected in MyClass and try to access it directly from outside the class:

<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function __toString()
    {
        echo "Using the toString method: ";
        return $this->getProperty();
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    protected function getProperty()
    {
        return $this->prop1 . "";
    }
}

class MyOtherClass extends MyClass
{
    public function __construct()
    {
        parent::__construct();
		echo "A new constructor in " . __CLASS__ . ".";
    }

    public function newMethod()
    {
        echo "From a new method in " . __CLASS__ . ".";
    }
}

// Create a new object
$newobj = new MyOtherClass;

// Attempt to call a protected method
echo $newobj->getProperty();
?>

Upon attempting to run this script, the following error shows up:

The class "MyClass" was initiated!
A new constructor in MyOtherClass.

Fatal error: Call to protected method MyClass::getProperty() from context '' in /Applications/XAMPP/xamppfiles/htdocs/testing/test.php on line 55

Now, create a new method in MyOtherClass to call the getProperty() method:

<?php

class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function __toString()
    {
        echo "Using the toString method: ";
        return $this->getProperty();
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    protected function getProperty()
    {
        return $this->prop1 . "";
    }
}

class MyOtherClass extends MyClass
{
    public function __construct()
    {
        parent::__construct();
		echo "A new constructor in " . __CLASS__ . ".";
    }

    public function newMethod()
    {
        echo "From a new method in " . __CLASS__ . ".";
    }

    public function callProtected()
    {
        return $this->getProperty();
    }
}

// Create a new object
$newobj = new MyOtherClass;

// Call the protected method from within a public method
echo $newobj->callProtected();
?>

This generates the desired result:

The class "MyClass" was initiated!
A new constructor in MyOtherClass.
I'm a class property!
The class "MyClass" was destroyed.

Private Properties and Methods

A property or method declared private is accessible only from within the class that defines it. This means that even if a new class extends the class that defines a private property, that property or method will not be available at all within the child class.

To demonstrate this, declare getProperty() as private in MyClass, and attempt to call callProtected() from MyOtherClass:

<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function __toString()
    {
        echo "Using the toString method: ";
        return $this->getProperty();
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    private function getProperty()
    {
        return $this->prop1 . "";
    }
}

class MyOtherClass extends MyClass
{
    public function __construct()
    {
        parent::__construct();
        echo "A new constructor in " . __CLASS__ . ".";
    }

    public function newMethod()
    {
        echo "From a new method in " . __CLASS__ . ".";
    }

    public function callProtected()
    {
        return $this->getProperty();
    }
}

// Create a new object
$newobj = new MyOtherClass;

// Use a method from the parent class
echo $newobj->callProtected();
?>

Reload your browser, and the following error appears:

The class "MyClass" was initiated!
A new constructor in MyOtherClass.

Fatal error: Call to private method MyClass::getProperty() from context 'MyOtherClass' in /Applications/XAMPP/xamppfiles/htdocs/testing/test.php on line 49

Static Properties and Methods

A method or property declared static can be accessed without first instantiating the class; you simply supply the class name, scope resolution operator, and the property or method name.

To demonstrate this, add a static property called $count and a static method called plusOne() to MyClass. Then set up a do…while loop to output the incremented value of $count as long as the value is less than 10:

<?php
class MyClass
{
    public $prop1 = "I'm a class property!";

    public static $count = 0;

    public function __construct()
    {
        echo 'The class "', __CLASS__, '" was initiated!';
    }

    public function __destruct()
    {
        echo 'The class "', __CLASS__, '" was destroyed.';
    }

    public function __toString()
    {
        echo "Using the toString method: ";
        return $this->getProperty();
    }

    public function setProperty($newval)
    {
        $this->prop1 = $newval;
    }

    private function getProperty()
    {
        return $this->prop1 . "";
    }

    public static function plusOne()
    {
        return "The count is " . ++self::$count . ".";
    }
}

class MyOtherClass extends MyClass
{
    public function __construct()
    {
        parent::__construct();
        echo "A new constructor in " . __CLASS__ . ".";
    }

    public function newMethod()
    {
        echo "From a new method in " . __CLASS__ . ".";
    }

    public function callProtected()
    {
        return $this->getProperty();
    }
}

do
{
    // Call plusOne without instantiating MyClass
    echo MyClass::plusOne();
} while ( MyClass::$count

Note — When accessing static properties, the dollar sign ($) comes after the scope resolution operator.

When you load this script in your browser, the following is output:

The count is 1.
The count is 2.
The count is 3.
The count is 4.
The count is 5.
The count is 6.
The count is 7.
The count is 8.
The count is 9.
The count is 10.

Commenting with DocBlocks

A DocBlock is defined by using a block comment that starts with an additional asterisk:

/**
 * This is a very basic DocBlock
 */

The real power of DocBlocks comes with the ability to use tags, which start with an at symbol (@) immediately followed by the tag name and the value of the tag. DocBlock tags allow developers to define authors of a file, the license for a class, the property or method information, and other useful information.

The most common tags used follow:

  • @author: The author of the current element (which might be a class, file, method, or any bit of code) are listed using this tag. Multiple author tags can be used in the same DocBlock if more than one author is credited. The format for the author name is John Doe <john.doe@email.com>.
  • @copyright: This signifies the copyright year and name of the copyright holder for the current element. The format is 2010 Copyright Holder.
  • @license: This links to the license for the current element. The format for the license information is
    http://www.example.com/path/to/license.txt License Name.
  • @var: This holds the type and description of a variable or class property. The format is type element description.
  • @param: This tag shows the type and description of a function or method parameter. The format istype $element_name element description.
  • @return: The type and description of the return value of a function or method are provided in this tag. The format is type return element description.

A sample class commented with DocBlocks might look like this:

<?php
/**
 * A simple class
 *
 * This is the long description for this class,
 * which can span as many lines as needed. It is
 * not required, whereas the short description is
 * necessary.
 *
 * It can also span multiple paragraphs if the
 * description merits that much verbiage.
 *
 * @author Author Name
 * @copyright 2011
 * @license http://www.php.net/license/3_01.txt PHP License 3.01
 */
class SimpleClass
{
    /**
     * A public variable
     *
     * @var string stores data for the class
     */
    public $foo;

    /**
     * Sets $foo to a new value upon class instantiation
     *
     * @param string $val a value required for the class
     * @return void
     */
    public function __construct($val)
    {
        $this->foo = $val;
    }

    /**
     * Multiplies two integers
     *
     * Accepts a pair of integers and returns the
     * product of the two.
     *
     * @param int $bat a number to be multiplied
     * @param int $baz a number to be multiplied
     * @return int the product of the two parameters
     */
    public function bar($bat, $baz)
    {
        return $bat * $baz;
    }
}

?>

Once you scan the preceding class, the benefits of DocBlock are apparent: everything is clearly defined so that the next developer can pick up the code and never have to wonder what a snippet of code does or what it should contain.

Comparing Object-Oriented and Procedural Code

There’s not really a right and wrong way to write code. That being said, this section outlines a strong argument for adopting an object-oriented approach in software development, especially in large applications.

Reason 1: Ease of Implementation

While it may be daunting at first, OOP actually provides an easier approach to dealing with data. Because an object can store data internally, variables don’t need to be passed from function to function to work properly.

Also, because multiple instances of the same class can exist simultaneously, dealing with large data sets is infinitely easier. For instance, imagine you have two people’s information being processed in a file. They need names, occupations, and ages.

The Procedural Approach

Here’s the procedural approach to our example:

<?php

function changeJob($person, $newjob)
{
    $person['job'] = $newjob; // Change the person's job
    return $person;
}

function happyBirthday($person)
{
    ++$person['age']; // Add 1 to the person's age
    return $person;
}

$person1 = array(
    'name' => 'Tom',
    'job' => 'Button-Pusher',
    'age' => 34
);

$person2 = array(
    'name' => 'John',
    'job' => 'Lever-Puller',
    'age' => 41
);

// Output the starting values for the people
echo "Person 1: ", print_r($person1, TRUE), "";
echo "Person 2: ", print_r($person2, TRUE), "";

// Tom got a promotion and had a birthday
$person1 = changeJob($person1, 'Box-Mover');
$person1 = happyBirthday($person1);

// John just had a birthday
$person2 = happyBirthday($person2);

// Output the new values for the people
echo "Person 1: ", print_r($person1, TRUE), "";
echo "Person 2: ", print_r($person2, TRUE), "";
?>

When executed, the code outputs the following:

Person 1: Array
(
    [name] => Tom
    [job] => Button-Pusher
    [age] => 34
)
Person 2: Array
(
    [name] => John
    [job] => Lever-Puller
    [age] => 41
)
Person 1: Array
(
    [name] => Tom
    [job] => Box-Mover
    [age] => 35
)
Person 2: Array
(
    [name] => John
    [job] => Lever-Puller
    [age] => 42
)

While this code isn’t necessarily bad, there’s a lot to keep in mind while coding. The array of the affected person’s attributes must be passed and returned from each function call, which leaves margin for error.

To clean up this example, it would be desirable to leave as few things up to the developer as possible. Only absolutely essential information for the current operation should need to be passed to the functions.

This is where OOP steps in and helps you clean things up.

The OOP Approach

Here’s the OOP approach to our example:

<?php
class Person
{
    private $_name;
    private $_job;
    private $_age;

    public function __construct($name, $job, $age)
    {
        $this->_name = $name;
        $this->_job = $job;
        $this->_age = $age;
    }

    public function changeJob($newjob)
    {
        $this->_job = $newjob;
    }

    public function happyBirthday()
    {
        ++$this->_age;
    }
}

// Create two new people
$person1 = new Person("Tom", "Button-Pusher", 34);
$person2 = new Person("John", "Lever Puller", 41);

// Output their starting point
echo "Person 1: ", print_r($person1, TRUE), "";
echo "Person 2: ", print_r($person2, TRUE), "";

// Give Tom a promotion and a birthday
$person1->changeJob("Box-Mover");
$person1->happyBirthday();

// John just gets a year older
$person2->happyBirthday();

// Output the ending values
echo "Person 1: ", print_r($person1, TRUE), "";
echo "Person 2: ", print_r($person2, TRUE), "";
?>

This outputs the following in the browser:

Person 1: Person Object
(
    [_name:private] => Tom
    [_job:private] => Button-Pusher
    [_age:private] => 34
)

Person 2: Person Object
(
    [_name:private] => John
    [_job:private] => Lever Puller
    [_age:private] => 41
)

Person 1: Person Object
(
    [_name:private] => Tom
    [_job:private] => Box-Mover
    [_age:private] => 35
)

Person 2: Person Object
(
    [_name:private] => John
    [_job:private] => Lever Puller
    [_age:private] => 42
)

Tip— Not everything needs to be object oriented. A quick function that handles something small in one place inside the application does not necessarily need to be wrapped in a class. Use your best judgment when deciding between object-oriented and procedural approaches.

Reason 2: Better Organization

Another benefit of OOP is how well it lends itself to being easily packaged and cataloged. Each class can generally be kept in its own separate file, and if a uniform naming convention is used, accessing the classes is extremely simple.

Assume you’ve got an application with 150 classes that are called dynamically through a controller file at the root of your application filesystem. All 150 classes follow the naming convention class.classname.inc.php and reside in the inc folder of your application.

The controller can implement PHP’s __autoload() function to dynamically pull in only the classes it needs as they are called, rather than including all 150 in the controller file just in case or coming up with some clever way of including the files in your own code:

<?php
    function __autoload($class_name)
    {
        include_once 'inc/class.' . $class_name . '.inc.php';
    }
?>

Having each class in a separate file also makes code more portable and easier to reuse in new applications without a bunch of copying and pasting.

Reason 3: Easier Maintenance

Due to the more compact nature of OOP when done correctly, changes in the code are usually much easier to spot and make than in a long spaghetti code procedural implementation.

If a particular array of information gains a new attribute, a procedural piece of software may require (in a worst-case scenario) that the new attribute be added to each function that uses the array.

An OOP application could potentially be updated as easily adding the new property and then adding the methods that deal with said property.

 
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Posted by on June 7, 2011 in Object Oriented PHP, PHP

 

Tags: ,

Object Oriented PHP for Beginners (Basic)

For this tutorial, you should understand a few PHP basics: functions, variables, conditionals and loops.

To make things easy, the tutorial is divided into 22 steps.

Step 1:
First thing we need to do is create two PHP pages:

  • index.php
  • class_lib.php

OOP is all about creating modular code, so our object oriented PHP code will be contained in dedicated files that we will then insert into our normal PHP page using php ‘includes’.

In this case, all our OO PHP code will be in the PHP file:

  • class_lib.php

OOP revolves around a construct called a ‘class’. Classes are the cookie-cutters / templates that are used to define objects.

Step 2:
Create a simple PHP class (in class_lib.php)

Instead of having a bunch of functions, variables and code floating around willy-nilly, to design your php scripts or code libraries the OOP way, you’ll need to define/create your own classes.

You define your own class by starting with the keyword ‘class’ followed by the name you want to give your new class.

<?php
class person 
{

}
?>

Note: You enclose a class using curly braces ( { } ) … just like you do with functions .

Step 3:
Add data to your class

Classes are the blueprints for php objects – more on that later. One of the big differences between functions and classes is that a class contains both data (variables) and functions that form a package called an: ‘object’.

When you create a variable inside a class, it is called a ‘property’.

<?php
class person 
{
    var $name;
}
?>

Note: The data/variables inside a class (ex: var name;) are called ‘properties’.

Step 4:
Add functions/methods to your class

In the same way that variables get a different name when created inside a class (they are called: properties,) functions also referred to (by nerds) by a different name when created inside a class – they are called ‘methods’.

A class’s methods are used to manipulate its own data / properties.

<?php
class person 
{
    var $name;
    function set_name($new_name) 
    {
        $this->name = $new_name;
    }
    function get_name() 
    {
        return $this->name;
    }
}
?>

Note: Don’t forget that in a class, variables are called ‘properties’ and functions are called ‘methods’.

Step 5:
Getter and setter functions

We’ve created two interesting functions/methods: get_name() and set_name().

These methods follow a common OOP convention that you see in many languages (including Java and Ruby) – where you create methods to ‘set’ and ‘get’ properties in a class.

Another convention is that getter and setter names should match the property names.

<?php
class person 
{
    var $name; 
    function set_name($new_name) 
    { 
        $this->name = $new_name;  
    }

    function get_name() 
    {
        return $this->name;
    }
} 
?>

Note: Notice that the getter and setter names, match the associated property name.

This way, when other PHP programmers want to use your objects, they will know that if you have a method/function called ‘set_name()’, there will be a property/variable called ‘name’.

Step 6:
The ‘$this’ variable

You probably noticed this line of code:

<?php
this->name = $new_name;

The $this is a built-in variable (built into all objects) which points to the current object. Or in other words, $this is a special self-referencing variable. You use $this to access properties and to call other methods of the current class.

<?php
function get_name() 
{
    return $this->name;
}

Note: This may be a bit confusing for some of you … that’s because you are seeing for the first time, one of those built in OO capabilities (built into PHP5 itself) that automatically does stuff for us.

For now, just think of $this as a special OO PHP keyword. When PHP comes across $this, the PHP engine knows what to do.

Step 7:
Use your class in your main PHP page. (index.php)

You would never create your PHP classes directly inside your main php pages – that would help defeat the purposes of object oriented PHP in the first place!

Instead, it is always best practice to create separate php pages that only contain your classes. Then you would access your php objects/classes by including them in your main php pages with either a php ‘include’ or ‘require’.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
	"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">

<head>
	<meta http-equiv="Content-Type" content="text/html;
	charset=iso-8859-1" />
	<title>OOP in PHP</title>
	<?php include("class_lib.php"); ?>

</head>
<body>
</body>
</html>

Note: Notice how we haven’t done anything with our class yet. We will do that next.

Step 8:
Instantiate/create your object

Classes are the blueprints/templates of php objects. Classes don’t actually become objects until you do something called: instantiation.

When you instantiate a class, you create an instance of it … thus creating the object.

In other words, instantiation is the process of creating an instance of an object in memory. What memory? The server’s memory of course!

<?php include("class_lib.php"); ?>

</head>
<body>
	<?php
		$stefan = new person();
	?>
</body>
</html>

Note: The variable $stefan becomes a handle/reference to our newly created person object. I call $stefan a ‘handle’, because we will use $stefan to control and use the person object.

If you run the PHP code now, you will not see anything displayed on your pages. The reason for this, is because we have not told PHP to do anything with the object we just created …

Step 9:
The ‘new’ keyword

To create an object out of a class, you need to use the ‘new’ keyword.

When creating/instantiating a class, you can optionally add brackets to the class name, as I did in the example below. To be clear, you can see in the code below how I can create multiple objects from the same class.

… From the PHP’s engine point of view, each object is its own entity. Does that make sense?

<?php include("class_lib.php"); ?>
</head>
<body>
	<?php
		$stefan = new person();
		$jimmy = new person;
	?>
</body>
</html>

Note: When creating an object, be sure not to quote the class name.

For example:

<?php
$stefan = new 'person';

… will give you an error.

Step 10:
Set an objects properties

Now that we’ve created/instantiated our two separate ‘person’ objects, we can set their properties using the methods (the setters) we created.

Please keep in mind that though both our person objects ($stefan and $jimmy) are based on the same ‘person’ class, as far as php is concerned, they are totally different objects.

<?php include("class_lib.php"); ?>
</head>

<body>
	<?php
		$stefan = new person();
		$jimmy = new person;
		$stefan->set_name("Stefan Mischook");
		$jimmy->set_name("Nick Waddles");
	?>
</body>
</html>

Step 11:
Accessing an object’s data

Now we use the getter methods to access the data held in our objects … this is the same data we inserted into our objects using the setter methods.

When accessing methods and properties of a class, you use the arrow (->) operator.

<?php include("class_lib.php"); ?>
</head>
<body>
	<?php
		 $stefan = new person();
		 $jimmy = new person;

		 $stefan->set_name("Stefan Mischook");
		 $jimmy->set_name("Nick Waddles");

		echo "Stefan's full name: " . $stefan->get_name();
		echo "Nick's full name: " . $jimmy->get_name(); 
	?>

</body>
</html>

Note: The arrow operator (->) is not the same operator used with associative arrays: =>.

Step 12:
Directly accessing properties – don’t do it!

You don’t have to use methods to access objects properties; you can directly get to them using the arrow operator (->) and the name of the variable.

For example: with the property $name (in object $stefan) you could get its value like so:

<?php
$name = $stefan->name;

Though doable, it is considered bad practice to do it because it can lead to trouble down the road. You should use getter methods instead – more on that later.

<?php include("class_lib.php"); ?>		
</head>		
<body>		
	<?php
		$stefan = new person();		
		$jimmy = new person;

		$stefan->set_name("Stefan Mischook");
		$jimmy->set_name("Nick Waddles");	

		// directly accessing properties in a class is a no-no.

		echo "Stefan's full name: ".$stefan->name;

	?>		

</body>
</html>

Step 13:
Constructors

All objects can have a special built-in method called a ‘constructor’. Constructors allow you to initialise your object’s properties (translation: give your properties values,) when you instantiate (create) an object.

Note: If you create a __construct() function (it is your choice,) PHP will automatically call the __construct() method/function when you create an object from your class.

The ‘construct’ method starts with two underscores (__) and the word ‘construct’.

<?php 		
	class person {
		var $name;
		function __construct($persons_name) {		
			$this->name = $persons_name;		
		}		

		function set_name($new_name) {
		 	 $this->name = $new_name;
		}	

		function get_name() {		
		 	 return $this->name;		
		 }		

	}	 	
?>

For the rest of this tutorial, I’m going to stop reminding you that:

Functions = methods
Variables = properties
Since this is an OO PHP tutorial, I will now use the OO terminology.

Step 14:
Create an object with a constructor

Now that we’ve created a constructor method, we can provide a value for the $name property when we create our person objects.

You ‘feed’ the constructor method by providing a list of arguments (like you do with a function) after the class name.

For example:

<?php
$stefan = new person("Stefan Mischook");

This saves us from having to call the set_name() method reducing the amount of code. Constructors are common and are used often in PHP, Java etc.

<?php include("class_lib.php"); ?>		
</head>		
<body>		
	<?php
		$stefan = new person("Stefan Mischook");
		echo "Stefan's full name: ".$stefan->get_name();
	?>		
</body>

</html>

This is just a tiny example of how the mechanisms built into OO PHP can save you time and reduce the amount of code you need to write. Less code means less bugs.

Step 15:
Restricting access to properties using ‘ access modifiers ‘

One of the fundamental principles in OOP is ‘encapsulation’. The idea is that you create cleaner better code, if you restrict access to the data structures (properties) in your objects.

You restrict access to class properties using something called ‘access modifiers’. There are 3 access modifiers:

public
private
protected
‘Public’ is the default modifier.

<?php 		
	class person {		
	var $name;		
		public $height;		
		protected $social_insurance;
		private $pinn_number;

		function __construct($persons_name) {		
			$this->name = $persons_name;		
		}		

		function set_name($new_name) {   	
			$this->name = $new_name;
		}	

		function get_name() {
			return $this->name;
		}		

	}
?>

Note: When you declare a property with the ‘var’ keyword, it is considered ‘public’.

Step 16:
Restricting access to properties: part 2

When you declare a property as ‘private’, only the same class can access the property.

When a property is declared ‘protected’, only the same class and classes derived from that class can access the property – this has to do with inheritance …more on that later.

Properties declared as ‘public’ have no access restrictions, meaning anyone can access them.

To help you understand this (probably) foggy aspect of OOP, try out the following code and watch how PHP reacts. Tip: read the comments in the code for more information:

<?php include("class_lib.php"); ?>  

</head>  
<body>
	<?php  
		$stefan = new person("Stefan Mischook");   
		echo "Stefan's full name: " .  $stefan->get_name() ;  

		/*  
		Since $pinn_number was declared private, this line of code 
		will generate an error. Try it out!   
		*/  

		echo "Tell me private stuff: ".$stefan->pinn_number;  
	?>  
</body>  

</html>

Note: If you try to access a private property/variable outside of the class, you will get this:

'Fatal error: Cannot access private property person::$pinn_number
in ...'

Step 17:
Restricting access to methods
Like properties, you can control access to methods using one of the three access modifiers:

public
protected
private
Why do we have access modifiers?

In a nutshell: it comes down to control -it is makes sense to control how people use classes.

The reasons for access modifiers and other OO constructs, can be tricky to understand … especially since we are just beginners here. So give yourself a chance!

That said, we can (summarize and) say that many OOP constructs exist with idea the many programmers may be working together on a project.

<?php
	class person {  
		var $name;  

		public $height;  
		protected $social_insurance;  
		private $pinn_number;

		function __construct($persons_name){   
		   $this->name = $persons_name;  
		}       

		private function get_pinn_number(){
			return
			$this->pinn_number;  
		}       
	}   
?>

Notes: Since the method get_pinn_number() is ‘private’, the only place you can use this method is in the same class – typically in another method. If you wanted to call/use this method directly in your PHP pages, you would need to declare it ‘public’.

Step 18:
Inheritance – reusing code the OOP way

Inheritance is a fundamental capability/construct in OOP where you can use one class, as the base/basis for another class … or many other classes.

Why do it?

Doing this allows you to efficiently reuse the code found in your base class.

Say, you wanted to create a new ’employee’ class … since we can say that ’employee’ is a type/kind of ‘person’, they will share common properties and methods.

… Making some sense?

In this type of situation, inheritance can make your code lighter … because you are reusing the same code in two different classes. But unlike ‘old-school’ PHP:

You only have to type the code out once.
The actual code being reused, can be reused in many (unlimited) classes but it is only typed out in one place … conceptually, this is sort-of like PHP includes().
Take a look at the sample PHP code:

// 'extends' is the keyword that enables inheritance
class employee extends person 
{
	function __construct($employee_name) {
		$this->set_name($employee_name);
	}
}

Step 19:
Reusing code with inheritance: part 2

Because the class ’employee’ is based on the class ‘person’, ’employee’ automatically has all the public and protected properties and methods of ‘person’.

Nerd note: Nerds would say that ’employee’ is a type of person.

The code:

class employee extends person 
{
	function __construct($employee_name){
		$this->set_name($employee_name);
	}
}

Notice how we are able to use set_name() in ’employee’, even though we did not declare that method in the ’employee’ class. That’s because we already created set_name() in the class ‘person’.

Nerd Note: the ‘person’ class is called (by nerds,) the ‘base’ class or the ‘parent’ class because it’s the class that the ’employee’ is based on. This class hierarchy can become important down the road when your projects become more complex.

Step 20:
Reusing code with inheritance: part 3

As you can see in the code snippet below, we can call get_name on our ’employee’ object, courtesy of ‘person’.

The code:

	<title>OOP in PHP</title>

	<?php include("class_lib.php"); ?>
</head>
<body>
	<?php 
		// Using our PHP objects in our PHP pages. 
		$stefan = new person("Stefan Mischook");
		echo "Stefan's full name: " . $stefan->get_name();

		$james = new employee("Johnny Fingers");
		echo "---> " . $james->get_name();
	?>

</body>
</html>

This is a classic example of how OOP can reduce the number of lines of code (don’t have to write the same methods twice) while still keeping your code modular and much easier to maintain.

Step 21:
Overriding methods

Sometimes (when using inheritance,) you may need to change how a method works from the base class.

For example, let’s say set_name() method in the ’employee’ class, had to do something different than what it does in the ‘person’ class.

You ‘override’ the ‘person’ classes version of set_name(), by declaring the same method in ’employee’.

The code snippet:

<?php
	class person {
		protected function set_name($new_name) {
			if ($new_name != "Jimmy Two Guns") {
				$this->name = strtoupper($new_name);
			}
		}
	} 

	class employee extends person {
		protected function set_name($new_name) {
			if ($new_name == "Stefan Sucks") {
				$this->name = $new_name;
			}
		}
	}
?>

Notice how set_name() is different in the ’employee’ class from the version found in the parent class: ‘person’.

Step 22:
Overriding methods: part 2

Sometimes you may need to access your base class’s version of a method you overrode in the derived (sometimes called ‘child’) class.

In our example, we overrode the set_name() method in the ’employee’ class. Now I’ve used this code:

<?php
person::set_name($new_name);

… to access the parent class’ (person) version of the set_name() method.

The code:

<?php 
	class person {
		// explicitly adding class properties are optional - but 
		// is good practice
		var $name;	 
		function __construct($persons_name) {
			$this->name = $persons_name;
		 }

		 public function get_name() {
		 	return $this->name;
		 }

		 // protected methods and properties restrict access to 
		 // those elements.
		 protected function set_name($new_name) {
		 	 if (name !=  "Jimmy Two Guns") {
		 	 	$this->name = strtoupper($new_name);
		 	 } 
		}
	} 

	// 'extends' is the keyword that enables inheritance
	class employee extends person {
	protected function set_name($new_name) {
		if ($new_name ==  "Stefan Sucks") {
			$this->name = $new_name;
		}
	 	 else if ($new_name == "Johnny Fingers") { person::set_name($new_name); }	 
	}

	function __construct($employee_name) {
		$this->set_name($employee_name);
	}
}
?>

Notes: Using the symbol:

::

… allows you to specifically name the class where you want PHP to search for a method:

'person::set_name()'

… tells PHP to search for set_name() in the ‘person’ class.

There is also a shortcut if you just want refer to current class’s parent – by using the ‘parent’ keyword.

The code:

protected function set_name($new_name) {	
	if ($new_name ==  "Stefan Sucks") {
		$this->name = $new_name;	
	 }	
	 else if ($new_name == "Johnny Fingers") { parent::set_name($new_name); }
}
 
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Posted by on June 6, 2011 in Object Oriented PHP, PHP