PHP 8.1.15 Released!

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

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:

It is not allowed to override a read-write property with a readonly property or vice versa.

<?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'

?>

Return Type Compatibility with Internal Classes

Prior to PHP 8.1, most internal classes or methods didn't declare their return types, and any return type was allowed when extending them.

As of PHP 8.1.0, most internal methods started to "tentatively" declare their return type, in that case the return type of methods should be compatible with the parent being extended; otherwise, a deprecation notice is emitted. Note that lack of an explicit return declaration is also considered a signature mismatch, and thus results in the deprecation notice.

If the return type cannot be declared for an overriding method due to PHP cross-version compatibility concerns, a ReturnTypeWillChange attribute can be added to silence the deprecation notice.

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
?>
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User Contributed Notes 8 notes

up
206
jackdracona at msn dot com
12 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
93
Mohammad Istanbouly
5 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
25
strata_ranger at hotmail dot com
12 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
akashwebdev at gmail dot com
7 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
10
jarrod at squarecrow dot com
13 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
1
niemans at pbsolo dot nl
3 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.
up
-3
Anonymous
4 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= '').
up
-2
sibian0218 at gmail dot com
4 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
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