PHP: Object Inheritance

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 5 notes

down

215

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
 
?>

down

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
}

down

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();
?>

down

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!

?>

down

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]

?>

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