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The Basics

The Basics

class

Basic class definitions begin with the
keyword , 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.

class

The class name can be any valid label, provided it is not a
PHP .
As of PHP 8.4.0, using a single underscore  as a
class name is deprecated.
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:
.

reserved word_``^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*$

A class may contain its
own , 
(called "properties"), and functions (called "methods").

constantsvariables

Voorbeeld: Simple Class definition

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

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

The pseudo-variable  is available when a
method is called from within an object context.
 is the value of the calling object.

$this``$this

Waarschuwing: > Calling a non-static method statically throws an . Prior to PHP 8.0.0, this would generate a deprecation notice, and would be undefined. Error``$this

Voorbeeld: Some examples of the 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()
    {
        A::foo();
    }
}

$a = new A();
$a->foo();

A::foo();

$b = new B();
$b->bar();

B::bar();
?>

$this is defined (A)

Deprecated: Non-static method A::foo() should not be called statically in %s  on line 27
$this is not defined.

Deprecated: Non-static method A::foo() should not be called statically in %s  on line 20
$this is not defined.

Deprecated: Non-static method B::bar() should not be called statically in %s  on line 32

Deprecated: Non-static method A::foo() should not be called statically in %s  on line 20
$this is not defined.

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

Readonly classes

 As of PHP 8.2.0, a class can be marked with the
  modifier.
 Marking a class as  will add the
 
 to every declared property, and prevent the creation of
 .
 Moreover, it is impossible to add support for them by using the
  attribute. Attempting to do so
 will trigger a compile-time error.
readonly modifierdynamic properties`AllowDynamicProperties`

<?php
#[\AllowDynamicProperties]
readonly class Foo {
}

// Fatal error: Cannot apply #[AllowDynamicProperties] to readonly class Foo
?>

 As neither untyped nor static properties can be marked with the
  modifier, readonly classes cannot declare
 them either:
`readonly`

<?php
readonly class Foo
{
    public $bar;
}

// Fatal error: Readonly property Foo::$bar must have type
?>

<?php
readonly class Foo
{
    public static int $bar;
}

// Fatal error: Readonly class Foo cannot declare static properties
?>

 A  class can be
 
 if, and only if, the child class is also a
  class.
extended

new

To create an instance of a class, the  keyword must
be used.  An object will always be created unless the object has a
 defined that throws an
 on error. Classes
should be defined before instantiation (and in some cases this is a
requirement).

newconstructorexception

If a variable containing a  with the name of a class is used with
, 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.

string``new

Opmerking: > If there are no arguments to be passed to the class's constructor, parentheses after the class name may be omitted.

Voorbeeld: Creating an instance

<?php
class SimpleClass {
}

$instance = new SimpleClass();
var_dump($instance);

// This can also be done with a variable:
$className = 'SimpleClass';
$instance = new $className(); // new SimpleClass()
var_dump($instance);
?>

As of PHP 8.0.0, using  with arbitrary expressions
is supported. This allows more complex instantiation if the expression
produces a . The expressions must be wrapped in parentheses.

new``string

Voorbeeld: Creating an instance using an arbitrary expression

 In the given example we show multiple examples of valid arbitrary expressions that produce a class name.
 This shows a call to a function, string concatenation, and the  constant.
`::class`

<?php

class ClassA extends \stdClass {}
class ClassB extends \stdClass {}
class ClassC extends ClassB {}
class ClassD extends ClassA {}

function getSomeClass(): string
{
    return 'ClassA';
}

var_dump(new (getSomeClass()));
var_dump(new ('Class' . 'B'));
var_dump(new ('Class' . 'C'));
var_dump(new (ClassD::class));
?>

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

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

new self``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
 it.

cloning

Voorbeeld: Object Assignment

<?php
class SimpleClass {
    public string $var;
}

$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);
?>

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

It's possible to create instances of an object in a couple of ways:

Voorbeeld: Creating new objects

<?php

class Test
{
    public static function getNew()
    {
        return new static();
    }
}

class Child extends Test {}

$obj1 = new Test(); // By the class name
$obj2 = new $obj1(); // Through the variable containing an object
var_dump($obj1 !== $obj2);

$obj3 = Test::getNew(); // By the class method
var_dump($obj3 instanceof Test);

$obj4 = Child::getNew(); // Through a child class method
var_dump($obj4 instanceof Child);

?>

bool(true)
bool(true)
bool(true)

It is possible to access a member of a newly created object in a single expression:

Voorbeeld: Access member of newly created object

<?php
echo (new DateTime())->format('Y'), PHP_EOL;

// surrounding parentheses are optional as of PHP 8.4.0
echo new DateTime()->format('Y'), PHP_EOL;
?>

2025
2025

Opmerking: > Prior to PHP 7.1, the arguments are not evaluated if there is no constructor function defined.

Properties and methods

Class properties and methods live in separate "namespaces", so it is possible to have a property and a method with the same name. Referring 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.

Voorbeeld: Property access vs. method call

<?php
class Foo
{
    public $bar = 'property';
    
    public function bar() {
        return 'method';
    }
}

$obj = new Foo();
echo $obj->bar, PHP_EOL, $obj->bar(), PHP_EOL;

property
method

That means that calling an  which has been assigned to a property is not directly
possible. Instead the property has to be assigned to a variable first, for
instance. It is possible to call such a property directly
by enclosing it in parentheses.

anonymous function

Voorbeeld: Calling an anonymous function stored in a property

<?php
class Foo
{
    public $bar;
    
    public function __construct() {
        $this->bar = function() {
            return 42;
        };
    }
}

$obj = new Foo();

echo ($obj->bar)(), PHP_EOL;

42

extends

A class can inherit the constants, methods, and properties of another class by
using the keyword  in the class
declaration. It is not possible to extend multiple classes; a
class can only inherit from one base class.

extends

The inherited constants, 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 or constant
as ,
they may not be overridden.  It is possible to access the overridden
methods or static properties by referencing them
with .

finalparent::

Opmerking: > As of PHP 8.1.0, constants may be declared as final.

Voorbeeld: Simple Class Inheritance

<?php
class SimpleClass
{
    function displayVar()
    {
        echo "Parent class\n";
    }
}

class ExtendClass extends SimpleClass
{
    // Redefine the parent method
    function displayVar()
    {
        echo "Extending class\n";
        parent::displayVar();
    }
}

$extended = new ExtendClass();
$extended->displayVar();
?>

Extending class
Parent Class

Signature compatibility rules

 When overriding a method, its signature must be compatible with the parent
 method. Otherwise, a fatal error is emitted, or, prior to PHP 8.0.0, an
  level error is generated.
 A signature is compatible if it respects the
  rules, makes a
 mandatory parameter optional, adds only optional new parameters and
 doesn't restrict but only relaxes the visibility.
 This is known as the Liskov Substitution Principle, or LSP for short.
 The ,
 and  methods are exempt from these signature
 compatibility rules, and thus won't emit a fatal error in case of a
 signature mismatch.
`E_WARNING`varianceconstructor`private`

Voorbeeld: Compatible child methods

<?php

class Base
{
    public function foo(int $a) {
        echo "Valid\n";
    }
}

class Extend1 extends Base
{
    function foo(int $a = 5)
    {
        parent::foo($a);
    }
}

class Extend2 extends Base
{
    function foo(int $a, $b = 5)
    {
        parent::foo($a);
    }
}

$extended1 = new Extend1();
$extended1->foo();
$extended2 = new Extend2();
$extended2->foo(1);

Valid
Valid

The following examples demonstrate that a child method which removes a parameter, or makes an optional parameter mandatory, is not compatible with the parent method.

Voorbeeld: Fatal error when a child method removes a parameter

<?php

class Base
{
    public function foo(int $a = 5) {
        echo "Valid\n";
    }
}

class Extend extends Base
{
    function foo()
    {
        parent::foo(1);
    }
}

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Voorbeeld: Fatal error when a child method makes an optional parameter mandatory

<?php

class Base
{
    public function foo(int $a = 5) {
        echo "Valid\n";
    }
}

class Extend extends Base
{
    function foo(int $a)
    {
        parent::foo($a);
    }
}

Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13

Waarschuwing: > Renaming a method's parameter in a child class is not a signature incompatibility. However, this is discouraged as it will result in a runtime if

  are used.
 `Error`named arguments

Voorbeeld: Error when using named arguments and parameters were renamed in a child class

<?php

class A {
    public function test($foo, $bar) {}
}

class B extends A {
    public function test($a, $b) {}
}

$obj = new B;

// Pass parameters according to A::test() contract
$obj->test(foo: "foo", bar: "bar"); // ERROR!

Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14
Stack trace:
#0 {main}
  thrown in /in/XaaeN on line 14

::class

The  keyword is also used for class
name resolution.
To obtain the fully qualified name of a class 
use . This is particularly useful with
 classes.

class``ClassName``ClassName::classnamespaced

Voorbeeld: Class name resolution

<?php
namespace NS {
    class ClassName {
    }
    
    echo ClassName::class;
}
?>

NS\ClassName

Opmerking: > The class name resolution using is a compile time transformation. That means at the time the class name string is created no autoloading has happened yet. As a consequence, class names are expanded even if the class does not exist. No error is issued in that case. ::class

Voorbeeld: Missing class name resolution

<?php
print Does\Not\Exist::class;
?>

Does\Not\Exist

As of PHP 8.0.0,  may also be used on
objects. This resolution happens at runtime, not compile time. Its effect is
the same as calling  on the object.

::class``get_class

Voorbeeld: Object name resolution

<?php
namespace NS {
    class ClassName {
    }

    $c = new ClassName();
    print $c::class;
}
?>

NS\ClassName

Nullsafe methods and properties

As of PHP 8.0.0, properties and methods may also be accessed with the "nullsafe" operator instead: . The nullsafe operator works the same as property or method access as above, except that if the object being dereferenced is null then null will be returned rather than an exception thrown. If the dereference is part of a chain, the rest of the chain is skipped. ?-&gt;

The effect is similar to wrapping each access in an check first, but more compact. is_null

Voorbeeld: Nullsafe Operator

<?php

// As of PHP 8.0.0, this line:
$result = $repository?->getUser(5)?->name;

// Is equivalent to the following code block:
if (is_null($repository)) {
    $result = null;
} else {
    $user = $repository->getUser(5);
    if (is_null($user)) {
        $result = null;
    } else {
        $result = $user->name;
    }
}
?>

Opmerking: > The nullsafe operator is best used when null is considered a valid and expected possible value for a property or method return. For indicating an error, a thrown exception is preferable.

Vorige ← Autoloading Classes Volgende OOP Changelog →

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