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# 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, 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 examples:

Example 19-1. $this variable in object-oriented language
`<?php class A { function foo() { if (isset($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 19-2. Simple Class definition
`<?php class SimpleClass { // member declaration public $var = 'a default value'; // method declaration public function displayVar() { echo $this->var; } } ?>`

> Note: There are some nice functions to handle classes and objects. You might want to take a look at the Class/Object Functions.

## 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 19-3. 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 19-4. 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 19-5. 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

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