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The Basics
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);
}
?>
There are some nice functions to handle classes and objects. You might want to take a look at the Class/Object Functions.
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).
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"
}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