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
?>PHP - Manual: 对象继承
2025-11-01
对象继承
继承已为大家所熟知的一个程序设计特性，PHP 的对象模型也使用了继承。继承将会影响到类与类，对象与对象之间的关系。
比如，当扩展一个类，子类就会继承父类所有 public 和 protected 的方法，属性和常量。除非子类覆盖了父类的方法，被继承的方法都会保留其原有功能。
继承有助于功能的设计和抽象，在实现类似的对象、增加新功能时，无须重复编写这些公用的功能。
子类无法访问父类的私有方法。因此，子类无需考虑正常的继承规则而重新实现私有方法。
然而，在 PHP 8.0.0 之前， final 和 static
的限制会应用于 private 方法。
从 PHP 8.0.0 开始，仅 private final 的构造器是唯一受限的 private 方法；
想要“禁用”构造器，我们通常用静态工厂方法作为代替。
方法，属性和常量的 可见性
可以放宽，例如 protected 方法可以标记为 public，
但不能增加限制，例如标记 public 属性为 private。有个例外是构造方法，可以限制其可见性，例如
public 构造方法可以在子类中标记为 private。
注意:
除非使用了自动加载，否则一个类必须在使用之前被定义。如果一个类扩展了另一个，则父类必须在子类之前被声明。此规则适用于类继承其它类与接口。
注意:
不允许使用只读属性覆盖可读写属性，反之亦然。
<?php
class A {
public int $prop;
}
class B extends A {
// Illegal: read-write -> readonly
public readonly int $prop;
}
?>
示例 #1 继承示例
<?php
class Foo
{
public function printItem($string)
{
echo 'Foo: ' . $string . PHP_EOL;
}
public function printPHP()
{
echo 'PHP is great.' . PHP_EOL;
}
}
class Bar extends Foo
{
public function printItem($string)
{
echo 'Bar: ' . $string . PHP_EOL;
}
}
$foo = new Foo();
$bar = new Bar();
$foo->printItem('baz'); // 输出: 'Foo: baz'
$foo->printPHP(); // 输出: 'PHP is great'
$bar->printItem('baz'); // 输出: 'Bar: baz'
$bar->printPHP(); // 输出: 'PHP is great'
?>返回类型与内部类兼容
PHP 8.1 之前，大多数内部类或方法没有声明其返回类型，并且在继承它们时允许返回任何类型。
自 PHP 8.1.0 起，大多数内部方法开始“暂时”声明其返回类型，在这种情况下，方法的返回类型应该与继承的父级方法兼容；否则，将发出弃用通知。注意，没有指定返回声明也会视为签名不匹配，从而导致弃用通知。
如果由于 PHP 跨版本兼容性问题而无法为重写方法声明返回类型，则可以添加 ReturnTypeWillChange 注解来消除弃用通知。
示例 #2 The overriding method does not declare any return type
<?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" as of PHP 8.1.0
?>示例 #3 The overriding method declares a wrong return type
<?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" as of PHP 8.1.0
?>示例 #4 The overriding method declares a wrong return type without a deprecation notice
<?php
class MyDateTime extends DateTime
{
/**
* @return DateTime|false
*/
#[\ReturnTypeWillChange]
public function modify(string $modifier) { return false; }
}
// No notice is triggered
?>
＋添加备注
用户贡献的备注 6 notes
jackdracona at msn dot com ¶
14 years ago
Mohammad Istanbouly ¶
7 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 ¶
9 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();
?>
jarrod at squarecrow dot com ¶
15 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
$lunch = new Cheddar; //works!
?>
strata_ranger at hotmail dot com ¶
14 years ago
I was recently extending a PEAR class when I encountered a situation where I wanted to call a constructor two levels up the class hierarchy, ignoring the immediate parent. In such a case, you need to explicitly reference the class name using the :: operator.
Fortunately, just like using the 'parent' keyword PHP correctly recognizes that you are calling the function from a protected context inside the object's class hierarchy.
E.g:
<?php
class foo
{
public function something()
{
echo __CLASS__; // foo
var_dump($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]
?>
ignacio at inek dot com dot ar ¶
1 month ago
In case you have a public readonly property in a class you need to extend, adding other properties, this can be a way to do it:
<?php
class A {
public function __construct(
public readonly int $prop
) {}
}
class B extends A {
public function __construct(
int $prop,
public readonly int $prop2
) {
parent::__construct($prop);
}
}
?>官方地址：https://www.php.net/manual/en/language.oop5.inheritance.php
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