ConFoo 2025

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, finalstatic 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

up
217
jackdracona at msn dot com
14 years ago
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

?>
up
103
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
}
up
21
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();
?>
up
20
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]

?>
up
14
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!

?>
up
-3
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= '').
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