PHP: Hypertext Preprocessor (original) (raw)

Object Inheritance

Inheritance is a well-established programming principle, and PHP makes use of this principle in its object model. This principle will affect the way many classes and objects relate to one another.

For example, when extending a class, the subclass inherits all of the public and protected methods, properties and constants from the parent class. Unless a class overrides those methods, they will retain their original functionality.

This is useful for defining and abstracting functionality, and permits the implementation of additional functionality in similar objects without the need to reimplement all of the shared functionality.

Private methods of a parent class are not accessible to a child class. As a result, child classes may reimplement a private method themselves without regard for normal inheritance rules. Prior to PHP 8.0.0, however, final and static restrictions were applied to private methods. As of PHP 8.0.0, the only private method restriction that is enforced is private final constructors, as that is a common way to "disable" the constructor when using static factory methods instead.

The visibility of methods, properties and constants can be relaxed, e.g. aprotected method can be marked aspublic, but they cannot be restricted, e.g. marking a public property as private. An exception are constructors, whose visibility can be restricted, e.g. a public constructor can be marked as private in a child class.

Note:

Unless autoloading is used, the classes must be defined before they are used. If a class extends another, then the parent class must be declared before the child class structure. This rule applies to classes that inherit other classes and interfaces.

Note:

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

<?phpclass A { public int $prop; } class B extends A { // Illegal: read-write -> readonly public readonly int $prop; } ?>

Example #1 Inheritance Example

`<?phpclass 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'); // Output: 'Foo: baz'
$foo->printPHP(); // Output: 'PHP is great'
$bar->printItem('baz'); // Output: 'Bar: baz'
$bar->printPHP(); // Output: '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.

Example #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 <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>m</mi><mi>o</mi><mi>d</mi><mi>i</mi><mi>f</mi><mi>i</mi><mi>e</mi><mi>r</mi><mo stretchy="false">)</mo><mi>s</mi><mi>h</mi><mi>o</mi><mi>u</mi><mi>l</mi><mi>d</mi><mi>e</mi><mi>i</mi><mi>t</mi><mi>h</mi><mi>e</mi><mi>r</mi><mi>b</mi><mi>e</mi><mi>c</mi><mi>o</mi><mi>m</mi><mi>p</mi><mi>a</mi><mi>t</mi><mi>i</mi><mi>b</mi><mi>l</mi><mi>e</mi><mi>w</mi><mi>i</mi><mi>t</mi><mi>h</mi><mi>D</mi><mi>a</mi><mi>t</mi><mi>e</mi><mi>T</mi><mi>i</mi><mi>m</mi><mi>e</mi><mo>:</mo><mo>:</mo><mi>m</mi><mi>o</mi><mi>d</mi><mi>i</mi><mi>f</mi><mi>y</mi><mo stretchy="false">(</mo><mi>s</mi><mi>t</mi><mi>r</mi><mi>i</mi><mi>n</mi><mi>g</mi></mrow><annotation encoding="application/x-tex">modifier) should either be compatible with DateTime::modify(string </annotation></semantics></math>modifier)shouldeitherbecompatiblewithDateTime::modify(stringmodifier): DateTime|false, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice" as of PHP 8.1.0 ?>

Example #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 <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>m</mi><mi>o</mi><mi>d</mi><mi>i</mi><mi>f</mi><mi>i</mi><mi>e</mi><mi>r</mi><mo stretchy="false">)</mo><mo>:</mo><mo stretchy="false">?</mo><mi>D</mi><mi>a</mi><mi>t</mi><mi>e</mi><mi>T</mi><mi>i</mi><mi>m</mi><mi>e</mi><mi>s</mi><mi>h</mi><mi>o</mi><mi>u</mi><mi>l</mi><mi>d</mi><mi>e</mi><mi>i</mi><mi>t</mi><mi>h</mi><mi>e</mi><mi>r</mi><mi>b</mi><mi>e</mi><mi>c</mi><mi>o</mi><mi>m</mi><mi>p</mi><mi>a</mi><mi>t</mi><mi>i</mi><mi>b</mi><mi>l</mi><mi>e</mi><mi>w</mi><mi>i</mi><mi>t</mi><mi>h</mi><mi>D</mi><mi>a</mi><mi>t</mi><mi>e</mi><mi>T</mi><mi>i</mi><mi>m</mi><mi>e</mi><mo>:</mo><mo>:</mo><mi>m</mi><mi>o</mi><mi>d</mi><mi>i</mi><mi>f</mi><mi>y</mi><mo stretchy="false">(</mo><mi>s</mi><mi>t</mi><mi>r</mi><mi>i</mi><mi>n</mi><mi>g</mi></mrow><annotation encoding="application/x-tex">modifier): ?DateTime should either be compatible with DateTime::modify(string </annotation></semantics></math>modifier):?DateTimeshouldeitherbecompatiblewithDateTime::modify(stringmodifier): DateTime|false, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice" as of PHP 8.1.0 ?>

Example #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
?>`

Improve This Page

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:

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

hello(); ?>

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:

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........

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=′′)shouldbecompatiblewithfoo::print(text2 = '') should be compatible with foo::print(text2=′′)shouldbecompatiblewithfoo::print(text= '').