class

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_\x80-\xff][a-zA-Z0-9_\x80-\xff]*$.

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

<?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 the value of the calling object.

<?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

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

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

<?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
?>

new

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 variable containing a string with 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.

Note:

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

<?php
$instance = new SimpleClass();

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

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

<?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 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);
?>

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:

<?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:

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

2016

Note: 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.

<?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 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. It is possible to call such a property directly by enclosing it in parentheses.

<?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 extends in the class declaration. It is not possible to extend multiple classes; a class can only inherit from one base class.

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 final, they may not be overridden. It is possible to access the overridden methods or static properties by referencing them with parent::.

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

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

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

Extending class
a default value

<?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

<?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

<?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

<?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 class keyword is also used for class name resolution. To obtain the fully qualified name of a class ClassName use ClassName::class. This is particularly useful with namespaced classes.

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

NS\ClassName

Note:

The class name resolution using ::class 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.

Example #16 Missing class name resolution

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

The above example will output:

Does\Not\Exist

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

<?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.

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

<?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;
    }
}
?>

Note:

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.