The Basics
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").
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 the value of the calling object.
Calling a non-static method statically throws an Error. Prior to PHP 8.0.0, this would generate a deprecation notice, and $this would be undefined.
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()
{
A::foo();
}
}
$a = new A();
$a->foo();
A::foo();
$b = new B();
$b->bar();
B::bar();
?>
Output of the above example in PHP 7:
$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.
Output of the above example in PHP 8:
$this is defined (A) Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27 Stack trace: #0 thrown in %s on line 27
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 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.
Note:
If there are no arguments to be passed to the class's constructor, parentheses after the class name may be omitted.
Example #3 Creating an instance
<?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.
Example #4 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 ::class constant.
<?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));
?>
Output of the above example in PHP 8:
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.
Example #5 Object Assignment
<?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"
}
It's possible to create instances of an object in a couple of ways:
Example #6 Creating new objects
<?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)
It is possible to access a member of a newly created object in a single expression:
Example #7 Access member of newly created object
<?php
echo (new DateTime())->format('Y');
?>
The above example will output something similar to:
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.
Example #8 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;
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. It is possible to call such a property directly by enclosing it in parentheses.
Example #9 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;
The above example will output:
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.
Example #10 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
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
E_WARNING level error is generated.
A signature is compatible if it respects the
variance rules, makes a
mandatory parameter optional, and if any new parameters are optional.
This is known as the Liskov Substitution Principle, or LSP for short.
The constructor,
and private methods are exempt from these signature
compatibility rules, and thus won't emit a fatal error in case of a
signature mismatch.
Example #11 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);
The above example will output:
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.
Example #12 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);
}
}
Output of the above example in PHP 8 is similar to:
Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13
Example #13 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);
}
}
Output of the above example in PHP 8 is similar to:
Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13
Renaming a method's parameter in a child class is not a signature incompatibility. However, this is discouraged as it will result in a runtime Error if named arguments are used.
Example #14 Error when using named arguments and parameters were renamed in a child class
<?php
class A {
public function test($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!
The above example will output something similar to:
Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14 Stack trace: #0 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.
Example #15 Class name resolution
<?php
namespace NS {
class ClassName {
}
echo ClassName::class;
}
?>
The above example will output:
NS\ClassName
Note:
The class name resolution using
::classis 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, the ::class constant 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.
Example #17 Object name resolution
<?php
namespace NS {
class ClassName {
}
}
$c = new ClassName();
print $c::class;
?>
The above example will output:
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.
Example #18 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;
}
}
?>
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.
