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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 10.1. $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 10.2. Simple Class definition

<?php
class SimpleClass
{
   
// member declaration
   
public $var = 'a default value';

   
// method declaration
   
public function displayVar() {
       echo
$this->var;
   }
}
?>


The default value must be a constant expression, not (for example) a variable, a class member or a function call.

Example 10.3. Class members' default value

<?php
class SimpleClass
{
   
// invalid member declarations:
   
public $var1 = 'hello '.'world';
   public
$var2 = <<<EOD
hello world
EOD;
   public
$var3 = 1+2;
   public
$var4 = self::myStaticMethod();
   public
$var5 = $myVar;

   
// valid member declarations:
   
public $var6 = myConstant;
   public
$var7 = self::classConstant;
   public
$var8 = array(true, false);
   
   
}
?>


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 a class, 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. Classes should be defined before instantiation (and in some cases this is a requirement).

Example 10.4. Creating an instance

<?php
$instance
= new SimpleClass();
?>


In the class context, it is possible to create a new object by new self and new parent.

When assigning an already created instance of a class 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 copy of an already created object can be made by cloning it.

Example 10.5. 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 with the same name defined in the parent class. It is possible to access the overridden methods or static members by referencing them with parent::

Example 10.6. Simple Class Inheritance

<?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