Basic class definitions begin with the keyword class, followed by a class name, followed by a pair of curly braces which enclose the definitions of the properties and methods belonging to the class.
The class name can be any valid label, provided it is not a PHP reserved word. A valid class name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: ^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$.
A class may contain its own constants, variables (called "properties"), and functions (called "methods").
Example #1 Simple Class definition
<?php
class SimpleClass
{
// property declaration
public $var = 'a default value';
// method declaration
public function displayVar() {
echo $this->var;
}
}
?>
The 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 possibly another object, if the method is called statically from the context of a secondary object).
Example #2 Some examples of the $this pseudo-variable
<?php
class A
{
function foo()
{
if (isset($this)) {
echo '$this is defined (';
echo get_class($this);
echo ")\n";
} else {
echo "\$this is not defined.\n";
}
}
}
class B
{
function bar()
{
// Note: the next line will issue a warning if E_STRICT is enabled.
A::foo();
}
}
$a = new A();
$a->foo();
// Note: the next line will issue a warning if E_STRICT is enabled.
A::foo();
$b = new B();
$b->bar();
// Note: the next line will issue a warning if E_STRICT is enabled.
B::bar();
?>
The above example will output:
$this is defined (A) $this is not defined. $this is defined (B) $this is not defined.
<?php
class SimpleClass
{
// property declaration
public $var = 'a default value';
// method declaration
public function displayVar() {
echo $this->var;
}
}
?>
<?php
class A
{
function foo()
{
if (isset($this)) {
echo '$this is defined (';
echo get_class($this);
echo ")\n";
} else {
echo "\$this is not defined.\n";
}
}
}
class B
{
function bar()
{
// Note: the next line will issue a warning if E_STRICT is enabled.
A::foo();
}
}
$a = new A();
$a->foo();
// Note: the next line will issue a warning if E_STRICT is enabled.
A::foo();
$b = new B();
$b->bar();
// Note: the next line will issue a warning if E_STRICT is enabled.
B::bar();
?>
The above example will output:
$this is defined (A) $this is not defined. $this is defined (B) $this is not defined.
To create an instance of a class, the new keyword must be used. An object will always be created 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).
If a string containing the name of a class is used with new, a new instance of that class will be created. If the class is in a namespace, its fully qualified name must be used when doing this.
<?php $instance = new SimpleClass(); // This can also be done with a variable: $className = 'SimpleClass'; $instance = new $className(); // 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.
<?php $instance = new SimpleClass(); $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"
}
PHP 5.3.0 introduced a couple of new ways to create instances of an object:
<?php
class Test
{
static public function getNew()
{
return new static;
}
}
class Child extends Test
{}
$obj1 = new Test();
$obj2 = new $obj1;
var_dump($obj1 !== $obj2);
$obj3 = Test::getNew();
var_dump($obj3 instanceof Test);
$obj4 = Child::getNew();
var_dump($obj4 instanceof Child);
?>
The above example will output:
bool(true) bool(true) bool(true)
Class properties and methods live in separate "namespaces", so it is possible to have a property and a method with the same name. Reffering to both a property and a method has the same notation, and whether a property will be accessed or a method will be called, solely depends on the context, i.e. whether the usage is a variable access or a function call.
class Foo
{
public $bar = 'property';
public function bar() {
return 'method';
}
}
$obj = new Foo();
echo $obj->bar, PHP_EOL, $obj->bar(), PHP_EOL;
The above example will output:
property method
That means that calling an anonymous function which has been assigned to a property is not directly possible. Instead the property has to be assigned to a variable first, for instance.
class Foo
{
public $bar;
public function __construct() {
$this->bar = function() {
return 42;
};
}
}
$obj = new Foo();
$func = $obj->bar;
echo $func(), PHP_EOL;
The above example will output:
42
A class can inherit the methods and properties of another class by using the keyword extends in the class declaration. It is not possible to extend multiple classes; a class can only inherit from one base class.
The inherited methods and properties can be overridden by redeclaring them with the same name defined in the parent class. However, if the parent class has defined a method as final, that method may not be overridden. It is possible to access the overridden methods or static properties by referencing them with parent::.
When overriding methods, the parameter signature should remain the same or PHP will generate an E_STRICT level error. This does not apply to the constructor, which allows overriding with different parameters.
<?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
Since PHP 5.5, the class keyword is also used for class name resolution. You can get a string containing the fully qualified name of the ClassName class by using ClassName::class. This is particularly useful with namespaced classes.
<?php
namespace NS {
class ClassName {
}
echo ClassName::class;
}
?>
The above example will output:
NS\ClassName
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