The Basics

class

Every class definition begins with the keyword class, followed by a class name, which can be any name that isn't a reserved word in PHP. Followed by a pair of curly braces, of which contains the definition of the classes members and methods. A pseudo-variable, $this is available when a method is called from within an object context. $this is a reference to the calling object (usually the object to which the method belongs, but can be another object, if the method is called statically from the context of a secondary object). This is illustrated in the following example:


<?php

class A

{

    function foo()

    {

        if (isset($this)) {

            echo '$this is defined (';

            echo get_class($this);

            echo ")\n";

        } else {

            echo "\$this is not defined.\n";

        }

    }

}



class B

{

    function bar()

    {

        A::foo();

    }

}



$a = new A();

$a->foo();

A::foo();

$b = new B();

$b->bar();

B::bar();

?>

The above example will output:

$this is defined (a)
$this is not defined.
$this is defined (b)
$this is not defined.

Example 19-1. Simple Class definition


<?php

class SimpleClass

{

    // member declaration

    public $var = 'a default value';



    // method declaration

    public function displayVar() {

        echo $this->var;

    }

}

?>

new

To create an instance of an object, a new object must be created and assigned to a variable. An object will always be assigned when creating a new object unless the object has a constructor defined that throws an exception on error.

Example 19-2. Creating an instance


<?php

$instance = new SimpleClass()

?>

When assigning an already created instance of an object to a new variable, the new variable will access the same instance as the object that was assigned. This behaviour is the same when passing instances to a function. A new instance of an already created object can be made by cloning it.

Example 19-3. Object Assignment


<?php

$assigned   =  $instance;

$reference  =& $instance;



$instance->var = '$assigned will have this value';



$instance = null; // $instance and $reference become null



var_dump($instance);

var_dump($reference);

var_dump($assigned);

?>

The above example will output:

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

extends

A class can inherit methods and members of another class by using the extends keyword in the declaration. It is not possible to extend multiple classes, a class can only inherit one base class.

The inherited methods and members can be overridden, unless the parent class has defined a method as final, by redeclaring them within the same name defined in the parent class. It is possible to access the overrided method or members by referencing them with parent::

Example 19-4. Simple Class Inherintance


<?php

class ExtendClass extends SimpleClass

{

    // Redefine the parent method

    function displayVar()

    {

        echo "Extending class\n";

        parent::displayVar();

    }

}



$extended = new ExtendClass();

$extended->displayVar();

?>

The above example will output:

Extending class
a default value