Here is some clarification about PHP inheritance – there is a lot of bad information on the net. PHP does support Multi-level inheritance. (I tested it using version 5.2.9). It does not support multiple inheritance.
This means that you cannot have one class extend 2 other classes (see the extends keyword). However, you can have one class extend another, which extends another, and so on.
Example:
<?php
class A {
// more code here
}
class B extends A {
// more code here
}
class C extends B {
// more code here
}
$someObj = new A(); // no problems
$someOtherObj = new B(); // no problems
$lastObj = new C(); // still no problems
?>
Наследование
Наследование — это хорошо зарекомендовавший себя принцип программирования, и PHP использует этот принцип в своей объектной модели. Этот принцип повлияет на то, как многие классы и объекты связаны друг с другом.
Например, при расширении класса дочерний класс наследует все общедоступные и защищённые методы, свойства и константы родительского класса. До тех пор пока эти методы не будут переопределены, они будут сохранять свою исходную функциональность.
Это полезно для определения и абстрагирования функциональности и позволяет реализовать дополнительную функциональность в похожих объектах без необходимости реализовывать всю общую функциональность.
Закрытые методы родительского класса недоступны для дочернего класса. В результате
дочерние классы могут повторно реализовать закрытый метод без учёта обычных
правил наследования. Однако до PHP 8.0.0 к закрытым методам применялись ограничения
final и static. Начиная с PHP 8.0.0,
единственное ограничение закрытого метода, которое применяется - это конструкторы private final,
поскольку это обычный способ "отключить" конструктор при использовании вместо него статичных фабричных методов.
Видимость методов,
свойств и констант можно ослабить, например, защищённый метод
может быть помечен как общедоступный, но нельзя ограничить видимость,
например, нельзя пометить общедоступное свойство как закрытое.
Исключением являются конструкторы, видимость которых может быть ограничена,
например, общедоступный конструктор может быть помечен
как закрытый в дочернем классе.
Замечание:
Если не используется автозагрузка, классы должны быть объявлены до того, как они будут использоваться. Если класс расширяет другой, то родительский класс должен быть объявлен до наследующего класса. Это правило применяется к классам, которые наследуют другие классы или интерфейсы.
Замечание:
Не разрешается переопределять свойство чтения-записи с помощью readonly-свойства или наоборот.
<?php
class A {
public int $prop;
}
class B extends A {
// Нельзя: read-write -> readonly
public readonly int $prop;
}
?>
Пример #1 Пример наследования
<?php
class Foo
{
public function printItem($string)
{
echo 'Foo: ' . $string . PHP_EOL;
}
public function printPHP()
{
echo 'PHP просто супер.' . PHP_EOL;
}
}
class Bar extends Foo
{
public function printItem($string)
{
echo 'Bar: ' . $string . PHP_EOL;
}
}
$foo = new Foo();
$bar = new Bar();
$foo->printItem('baz'); // Выведет: 'Foo: baz'
$foo->printPHP(); // Выведет: 'PHP просто супер'
$bar->printItem('baz'); // Выведет: 'Bar: baz'
$bar->printPHP(); // Выведет: 'PHP просто супер'
?>Совместимость типов возвращаемых значений с внутренними классами
До PHP 8.1.0 большинство внутренних классов или методов не объявляли свои типы возвращаемых значений и при их расширении допускался любой тип возвращаемого значения.
Начиная с PHP 8.1.0, большинство внутренних методов начали "предварительно" объявлять тип возвращаемого значения. В этом случае тип возвращаемого значения методов должен быть совместим с расширяемым родителем; в противном случае выдаётся уведомление об устаревании. Обратите внимание, что отсутствие явного объявления типа возвращаемого значения также считается несоответствием сигнатуры и, соответственно, приводит к уведомлению об устаревании.
Если тип возвращаемого значения не может быть объявлен для переопределяемого метода из-за проблем с совместимостью с различными версиями PHP, может быть добавлен атрибут ReturnTypeWillChange, чтобы заглушить уведомление об устаревании.
Пример #2 Переопределяющий метод не объявляет никакого типа возвращаемого значения
<?php
class MyDateTime extends DateTime
{
public function modify(string $modifier) { return false; }
}
// "Deprecated: Return type of MyDateTime::modify(string $modifier) should either be compatible with DateTime::modify(string $modifier): DateTime|false, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice", начиная с PHP 8.1.0
?>Пример #3 Переопределяющий метод объявляет неверный тип возвращаемого значения
<?php
class MyDateTime extends DateTime
{
public function modify(string $modifier): ?DateTime { return null; }
}
// "Deprecated: Return type of MyDateTime::modify(string $modifier): ?DateTime should either be compatible with DateTime::modify(string $modifier): DateTime|false, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice", начиная с PHP 8.1.0
?>Пример #4 Переопределяющий метод объявляет неверный тип возвращаемого значения без уведомления об устаревании
<?php
class MyDateTime extends DateTime
{
/**
* @return DateTime|false
*/
#[\ReturnTypeWillChange]
public function modify(string $modifier) { return false; }
}
// Уведомление об устаревании не выводится
?>
add a note
User Contributed Notes 18 notes
jackdracona at msn dot com ¶
14 years ago
Mohammad Istanbouly ¶
6 years ago
I think the best way for beginners to understand inheritance is through a real example so here is a simple example I can gave to you
<?php
class Person
{
public $name;
protected $age;
private $phone;
public function talk(){
//Do stuff here
}
protected function walk(){
//Do stuff here
}
private function swim(){
//Do stuff here
}
}
class Tom extends Person
{
/*Since Tom class extends Person class this means
that class Tom is a child class and class person is
the parent class and child class will inherit all public
and protected members(properties and methods) from
the parent class*/
/*So class Tom will have these properties and methods*/
//public $name;
//protected $age;
//public function talk(){}
//protected function walk(){}
//but it will not inherit the private members
//this is all what Object inheritance means
}
akashwebdev at gmail dot com ¶
8 years ago
The Idea that multiple inheritence is not supported is correct but with tratits this can be reviewed.
for e.g.
<?php
trait custom
{
public function hello()
{
echo "hello";
}
}
trait custom2
{
public function hello()
{
echo "hello2";
}
}
class inheritsCustom
{
use custom, custom2
{
custom2::hello insteadof custom;
}
}
$obj = new inheritsCustom();
$obj->hello();
?>
strata_ranger at hotmail dot com ¶
13 years ago
I was recently extending a PEAR class when I encountered a situation where I wanted to call a constructor two levels up the class hierarchy, ignoring the immediate parent. In such a case, you need to explicitly reference the class name using the :: operator.
Fortunately, just like using the 'parent' keyword PHP correctly recognizes that you are calling the function from a protected context inside the object's class hierarchy.
E.g:
<?php
class foo
{
public function something()
{
echo __CLASS__; // foo
var_dump($this);
}
}
class foo_bar extends foo
{
public function something()
{
echo __CLASS__; // foo_bar
var_dump($this);
}
}
class foo_bar_baz extends foo_bar
{
public function something()
{
echo __CLASS__; // foo_bar_baz
var_dump($this);
}
public function call()
{
echo self::something(); // self
echo parent::something(); // parent
echo foo::something(); // grandparent
}
}
error_reporting(-1);
$obj = new foo_bar_baz();
$obj->call();
// Output similar to:
// foo_bar_baz
// object(foo_bar_baz)[1]
// foo_bar
// object(foo_bar_baz)[1]
// foo
// object(foo_bar_baz)[1]
?>
jarrod at squarecrow dot com ¶
14 years ago
You can force a class to be strictly an inheritable class by using the "abstract" keyword. When you define a class with abstract, any attempt to instantiate a separate instance of it will result in a fatal error. This is useful for situations like a base class where it would be inherited by multiple child classes yet you want to restrict the ability to instantiate it by itself.
Example........
<?php
abstract class Cheese
{
//can ONLY be inherited by another class
}
class Cheddar extends Cheese
{
}
$dinner = new Cheese; //fatal error
$lunch = new Cheddar; //works!
?>
niemans at pbsolo dot nl ¶
4 years ago
Inheritance works at create time, i.e. using the keyword 'new'. Static properties confused my understanding, so in order tho show the effect of visibility to inherintence I've created a simple demo script along with some set and get magic:
<?php
class A {
private $a = 'private';
protected $b = 'protected';
public $c = 'public';
static $d = 'static';
public function __construct()
{
$this->e = 'constructed';
}
public function __set($property, $value)
{
echo ' set ' . $property . '=' . $value;
$this->$property=$value;
}
public function __get($property)
{
echo ' get ' . $property;
$this->$property = 'dynamic'; // invokes __set() !!
return $this->$property;
}
}
class B extends A
{
public function constructMe()
{
$this->e = 'constructed2';
}
}
class C extends B
{
public function __construct()
{
parent::constructMe();
}
}
echo " \n";
$a = new A();
$b = new B();
echo " \n";
echo ' B:c='.$b->c;
echo " \n";
echo ' B:d=' .$b->d;
echo " \n";
$c = new C();
echo " \n";
print_r($a);
print_r($b);
print_r($c);
print_r(A::$d);
print_r(B::$d);
print_r(C::$d);
echo 'A class: ';
$R = new reflectionclass('A');
print_r($R->getdefaultproperties());
print_r($R->getstaticproperties());
echo 'B class: ';
$R = new reflectionclass('B');
print_r($R->getdefaultproperties());
print_r($R->getstaticproperties());
?>
This outputs:
set e=constructed
B:c=public
get d set d=dynamic B:d=dynamic
set e=constructed2
A Object
(
[a:A:private] => private
[b:protected] => protected
[c] => public
[e] => constructed
)
B Object
(
[a:A:private] => private
[b:protected] => protected
[c] => public
[d] => dynamic
)
C Object
(
[a:A:private] => private
[b:protected] => protected
[c] => public
[e] => constructed2
)
staticstaticstaticA class: Array
(
[d] => static
[a] => private
[b] => protected
[c] => public
)
Array
(
[d] => static
)
B class: Array
(
[d] => static
[b] => protected
[c] => public
)
Array
(
[d] => static
)
This shows how private variables ($a) are inherited, how static variables ($d) are inherited (by the class, not by the object) and that changing or adding variables in the parent ($e, $d) are not inherited by the child.
nemanja ¶
6 years ago
Even when autoloading (SPL) is used, class inheritance does not seem to work. Simply the PHP engine is unable to find parent (inherited) class. PHP 5.6 and 7.0 behave exactly same on this, which beats the purpose of autoloading.
And IMHO it's easy to fix as the autoloader is able to find all first level classes w/o problems, it just needs to follow same path recursively on parents too.
<?php
//Using default SPL autoloader, with namespaces mapping 1:1 to directory structure, with file names being all lowercase.
//This works with first level classes only, for inheritance it does NOT work, it cannot find parent classes.
spl_autoload_register();
//This is ugly but working code if you want to be able to autoload parent classes too.
spl_autoload_register(function ($class){
require_once __DIR__ . '/' . strtolower(str_replace('\\', '/', $class) . '.php');
});
gino dot d at amail dot com ¶
7 years ago
A simple trait to create multi inheritance in php
<?php
trait tExtend {
private $objectExtend = [];
final public function extend() {
foreach ($objects = func_get_args() as &$object) {
if (is_object($object) && !$object instanceOf self) {
$this->objectExtend[] = $object;
if (is_callable([$object, 'syncExtend']))
$object->syncExtend($this, $objects);
}
}
}
final public function syncExtend(&$object = null, array $objects) {
if (is_object($object) && !$object instanceOf self && !in_array($object, $this->objectExtend))
$this->objectExtend[] = $object;
foreach ($objects as &$object)
if (is_object($object) && !$object instanceOf self && !in_array($object, $this->objectExtend))
$this->objectExtend[] = $object;
return $this;
}
final public function __call($method, $args) {
if (method_exists($this, $method))
return $this->{$method}(... $args);
else {
foreach ($this->objectExtend as $i => &$object) {
if (method_exists($object, $method))
return $object->{$method}(... $args);
}
}
throw new \Exception('Call to undefined method ' . get_called_class() . '::' . $method . '()');
}
final public function __get($key) {
if (array_key_exists($key, $this))
return $this->{$key};
foreach ($this->objectExtend as &$object) {
if (array_key_exists($key, $object))
return $object->{$key};
}
trigger_error('Undefined property: ' . get_called_class() . '::$' . $key, E_USER_NOTICE);
}
final public function __isset($key) {
if (array_key_exists($key, $this))
return true;
foreach ($this->objectExtend as &$object) {
if (array_key_exists($key, $object))
return true;
}
return false;
}
}
/*************/
class F {
public $relationship = 'Sister';
}
class M {
public $relationship = 'Brother';
}
class A extends M {
use tExtend;
}
class B extends F {
use tExtend;
function getName() {
return 'B';
}
}
class C extends F {
use tExtend;
function getName() {
return 'C';
}
}
class D extends M {
use tExtend;
}
class Z {
use tExtend;
function getParent() {
return 'Z';
}
function __construct() {
$this->extend(... func_get_args());
}
}
$a = new A;
$b = new B;
$c = new C;
$d = new D;
$z = new Z($a, $b, $c, $d);
$d->extend($a); // Change nothing
$a->varA = 'A';
$c->varA = 'C';
echo $a->getParent(); // print Z
echo $a->getName(); // print B
echo $b->getName(); // print B
echo $c->getName(); // print C
echo $d->getName(); // print B
echo $z->getName(); // print B
echo $z->varA; // print A
echo $c->varA; // print C
?>
shaun at slickdesign dot com dot au ¶
6 years ago
Similar to functions, unless they are in conditionals, it is possible to define classes anywhere within your script and they still generate instances.
It is also possible to define child classes before the parent class, however, this seems to act the same way as a conditional, meaning that you need to define these classes before they can be used.
Examples:
- Standard usage (A and B are always accessible)
<?php
class A {}
class B extends A {}
var_dump( class_exists( 'A' ), class_exists( 'B' ) );
// Outputs: bool(true) bool(true)
?>
- End of file declaration (A and B are still always accessible, even if you have a return or exit before it)
<?php
var_dump( class_exists( 'A' ), class_exists( 'B' ) );
// Outputs: bool(true) bool(true)
die;
class A {}
class B extends A {}
?>
- Child before parent declaration (B is not accessible until it has been defined)
<?php
var_dump( class_exists( 'A' ), class_exists( 'B' ) );
// Outputs: bool(true) bool(false)
class B extends A {}
var_dump( class_exists( 'B' ) );
// Outputs: bool(true)
class A {}
?>
- Conditional definition (Same as child before parent example above)
<?php
var_dump( class_exists( 'A' ), class_exists( 'B' ) );
// Outputs: bool(true) bool(false)
if ( ! class_exists( 'B' ) ) {
class B extends A {}
}
var_dump( class_exists( 'B' ) );
// Outputs: bool(true)
class A {}
Sam Tigall ¶
4 years ago
Overriding a method which is called from base class works like this:
<?php
class Foo
{
public function printItem($string)
{
echo 'Foo: ' . $string;
}
public function printPHP($string)
{
$this->printItem($string);
}
}
class Bar extends Foo
{
public function printItem($string)
{
echo 'Bar: ' . $string ;
}
}
$foo = new Foo();
$bar = new Bar();
$foo->printPHP('baz'); //Foo: baz
$bar->printPHP('baz'); //Bar: baz
?>
Anonymous ¶
5 years ago
PHP7 gives you a warning if you redeclare a function in a child class with different parameters. For example:
class foo {
function print($text='') {
print text;
}
}
class bar extends foo {
function print($text1='',$text2='') {
print text1.text2
}
}
will give a PHP Warning: Declaration of bar::print($text1 = '', $text2 = '') should be compatible with foo::print($text= '').
sibian0218 at gmail dot com ¶
5 years ago
I've noticed one thing concerning inheritance...
When declaring an abstract class with a private method,
which is overridden by a sub-class, private takes precedence over public for child class...
(in the case you're redeclaring a method with a different signature in fact).
Hope this helps
ghoucine at gmail dot com ¶
9 years ago
For multiple single inheretance to work the order of class definition is very important if you're going up more than two levels in inheretence.
This won't work:
<?php
class A extends B {}
class B extends C {}
class C {}
$A = new A;
?>
Running the code above will spit a fatal error (Fatal Error:Class 'B' not found)
A change in class definition order will fix this:
<?php
class A extends B {}
class C {}
class B extends C {}
$A = new A;
?>
If you're goin up just one level in inhertance though there shouldn't be any problem. So this code works too:
<?php
class A extends B {}
class B {}
$A = new A;
?>
I have no explanation as to why this is the case though.
tim at web dot com ¶
6 years ago
[Document:Main.php]
<?php
//
include("person.php");
include("student.php");
include("teacher.php");
//-----Creating a object of the class "teacher"--------
$t1=new teacher();
$t1->name="Mustermann";
$t1->forename="Max";
$t1->email="max@mustermann.com";
echo $t1->name.", ".$t1->forename.": ".$t1->email;
$t1->ID();
//-----Creating a object of the class "student"--------
$s1=new student();
$s1->name="Kevin";
$s1->forename="Alpha";
echo $s1->name."; ".$s1->forename;
$s1->ID();
?>
//-------------------------------------------------------------------------------------------
[Document:person.php]
<?php
class person
{
//Attribute
public $name;
public $forename;
}
?>
//-------------------------------------------------------------------------------------------
[Document:teacher.php]
<?php
class teacher extends person /
{
//Attribute
public $email;
//Function
public function ID()
{
echo "<p>I am a teacher</p>";
}
}
?>
//-------------------------------------------------------------------------------------------
[Document:student.php]
<?php
class student extends Person
{
//Attribute
//Functionen
public function ID()
{
echo"<p>I am a student</p>";
}
}
?>
OZ ¶
13 years ago
Model for Mixins pattern:
<?php
interface IMixinsCaller
{
public function __mixin_get_property($property);
public function __mixin_set_property($property, $value);
public function __mixin_call($method, $value);
}
abstract class MixinsCaller implements IMixinsCaller
{
protected $mixins = array();
public function __call($name, $arguments)
{
if (!empty($this->mixins))
{
foreach ($this->mixins as $mixin)
{
if (method_exists($mixin, $name))
{
return call_user_func_array(array($mixin, $name), $arguments);
}
}
}
trigger_error('Non-existent method was called in class '.__CLASS__.': '.$name, E_USER_WARNING);
}
public function __mixin_get_property($property)
{
if (property_exists($this, $property))
{
return $this->$property;
}
trigger_error('Non-existent property was get in class '.__CLASS__.': '.$property, E_USER_WARNING);
}
public function __mixin_set_property($property, $value)
{
if (property_exists($this, $property))
{
return $this->$property = $value;
}
trigger_error('Non-existent property was set in class '.__CLASS__.': '.$property, E_USER_WARNING);
}
public function __mixin_call($method, $value)
{
if (method_exists($this, $method))
{
return call_user_func_array(array($this, $method), $value);
}
trigger_error('Non-existent method was called in class '.__CLASS__.': '.$method, E_USER_WARNING);
}
public function AddMixin($mixin)
{
$this->mixins[] = $mixin;
}
}
abstract class Mixin
{
/** @var IMixinsCaller $parent_object */
private $parent_object;
public function __construct(IMixinsCaller $parent_object)
{
$this->parent_object = $parent_object;
}
public function __get($property)
{
return $this->parent_object->__mixin_get_property($property);
}
public function __set($property, $value)
{
return $this->parent_object->__mixin_set_property($property, $value);
}
public function __call($method, $value)
{
return $this->parent_object->__mixin_call($method, $value);
}
}
?>
clintonabco at gmail dot com ¶
6 years ago
This page is not clear about the nature and specifics of Object Inheritance especially that you can only make the visibility of an inherited method or property weaker and not stronger within the subclass that is inheriting these from its parent class.
Example:
<?php
class Test1
{
protected $a_property = "This is a property";
protected function printProperty()
{
echo $this->a_property;
}
}
class Test2 extends Test1
{
private $b_property = "This is another property";
private function printProperty()
{
echo $this->b_property;
}
}
$test2 = new Test2();
?>
This code produces a PHP Fatal error with message:
PHP Fatal error: Access level to Test2::printProperty() must be protected (as in class Test1) or weaker
So if you inherit a protected method and you want redeclare it in your subclass then you can either declare its visibility as protected or public. If public is inherited then it stays public.
Here is the working version of the code snippet above:
<?php
class Test1
{
protected $a_property = "This is a property";
protected function printProperty()
{
echo $this->a_property;
}
}
class Test2 extends Test1
{
private $b_property = "This is another property";
public function printProperty()
{
echo $this->b_property;
}
}
$test2 = new Test2();
$test2->printProperty();
?>
To iterate what the description on the page says, subclasses inherit all protected and public properties and methods.
php at sleep is the enemy dot co dot uk ¶
14 years ago
Here's fun, an attempt to make some degree of multiple inheritance work in PHP using mixins. It's not particularly pretty, doesn't support method visibility modifiers and, if put to any meaningful purpose, could well make your call stack balloon to Ruby-on-Rails-esque proportions, but it does work.
<?php
abstract class Mix {
protected $_mixMap = array();
public function __construct(){
$this->_mixMap = $this->collectMixins($this);
}
public function __call($method, $args){
// doesn't pass scope
//return call_user_func_array(array($className, $method), $args);
// Error: Given object is not an instance of the class this method was declared in
//$method = new ReflectionMethod($className, $method);
//return $method->invokeArgs($this, $args);
$payload = $this->buildMixinPayload($this->_mixMap, $method, $args);
if(!$payload) throw new Exception('Method ' . $method . ' not found');
list($mixinMethod, list($method, $args)) = $payload;
return $this->$mixinMethod($method, $args);
}
protected function collectMixins($class){
static $found = array();
static $branch = array();
if(empty($branch)) $branch[] = get_class($this);
$mixins = array();
foreach(array_reverse(get_class_methods($class)) as $method){
if(preg_match('/^mixin(\w+)$/', $method, $matches)){
$className = $matches[1];
if(in_array($className, $branch))
throw new Exception('Circular reference detected ' . implode(' > ', $branch) . ' > ' . $className);
if(!in_array($className, $found)){
if(!class_exists($className)) throw new Exception('Class ' . $className . ' not found');
// populate props from mixin class
foreach(get_class_vars($className) as $key => $value){
if(!property_exists($this, $key)) $this->$key = $value;
}
$found[] = $branch[] = $className;
$mixins[$className] = $this->collectMixins($className);
}
$branch = array(get_class($this));
}
}
return $mixins;
}
protected function buildMixinPayload($mixins, $method, $args){
foreach($mixins as $className => $parents){
$mixinMethod = 'mixin' . $className;
if(method_exists($className, $method)) return array($mixinMethod, array($method, $args));
if(!empty($parents) && $return = $this->buildMixinPayload($parents, $method, $args)){
return array($mixinMethod, $return);
}
}
return false;
}
}
?>
msg2maciej at aol dot com ¶
13 years ago
PHP supports single class inheritance. My bare idea on accessing protected methods with power of abstracts and sort of "multi-class inheritance SIMULATION":
<?php
error_reporting(E_ALL);
abstract class Base {
abstract protected function __construct ();
abstract protected function hello_left ();
abstract protected function hello_right ();
}
abstract class NotImplemented_Left extends Base {
protected function hello_right () {
echo 'well, wont see that'; }}
abstract class NotImplemented_Right extends Base {
protected function hello_left () {
echo 'well, wont see that'; }}
class Left extends NotImplemented_Left {
protected function __construct () { # limited visibility, no access from "outside"
echo __CLASS__.'::protected __construct'. "\n"; }
protected function hello_left () { # limited visibility, no access from "outside"
echo 'protected hello_left in ' . __CLASS__ . "\n"; }}
class Right extends NotImplemented_Right {
protected function __construct () { # limited visibility, no access from "outside"
echo __CLASS__.'::protected __construct'. "\n"; }
protected function hello_right () { # limited visibility, no access from "outside"
echo 'protected hello_right in ' . __CLASS__ . "\n"; }
protected function hello_left () {
echo "wont see that, and easy to get rid of it from here\n"; }}
class Center extends Base {
private $left;
private $right;
public function __construct () {
echo 'welcome in ' . __CLASS__ . "\n";
echo 'Center::'; $this->left = new Left;
echo 'Center::'; $this->right = new Right;
echo " oh and\n";
$this->hello_left();
$this->hello_right();
}
public function hello_left () { # calling class Left
echo __CLASS__.'::'; $this->left->hello_left(); }
public function hello_right () { # calling class Right
echo __CLASS__.'::'; $this->right->hello_right(); }
}
$c = new Center;
?>
Produces:
welcome in Center
Center::Left::protected __construct
Center::Right::protected __construct
oh and
Center::protected hello_left in Left
Center::protected hello_right in Right
