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;
    }
}
?>

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

Example 19-3. 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(truefalse);
    
    
}
?>

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 19-4. Creating an instance

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

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 19-5. 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 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 19-6. 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