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. a protected method can be marked as public, 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.

    <?php

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

    Example #1 Inheritance Example

    <?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'); // 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 $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
    ?>

    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 $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
    ?>

    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
    ?>

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    User Contributed Notes 6 notes

    up
    214
    jackdracona at msn dot com
    15 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
    102
    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
    19
    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
    15
    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
    17
    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
    1
    ignacio at inek dot com dot ar
    4 months 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);
    }
    }
    ?>
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