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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, of which contains the definition of the classes members and methods. Within each method, except for static methods, a pseudo variable $this is available. $this is a reference to the same instance that called the method.

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 overloaded, 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 overloaded 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
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