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 you extend a class, the subclass inherits all of the public and protected methods 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.

Note:
Unless autoloading is used, then 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.

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'

?>

User Contributed Notes

jackdracona at msn dot com

6 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
 
?>

strata_ranger at hotmail dot com

5 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]

?>

jarrod at squarecrow dot com

6 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!

?>

akashwebdev at gmail dot com

10 months 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();
?>

ghoucine at gmail dot com

1 year ago


For multiple single inheretance to work the order of class definition is very important if you're going up more than two levels in inheretence.

This won't work:

<?php

class A extends B {}

class B extends C {}

class C {}

$A = new A; 
?>
Running the code above will spit a fatal error (Fatal Error:Class 'B' not found)

A change in class definition order will fix this:

<?php

class A extends B {}

class C {}

class B extends C {}

$A = new A;
?>

If you're goin up just one level in inhertance though there shouldn't be any problem. So this code works too:

<?php

class A extends B {}

class B  {}

$A = new A;

?>

I have no explanation as to why this is the case though.

php at sleep is the enemy dot co dot uk

6 years ago


Here's fun, an attempt to make some degree of multiple inheritance work in PHP using mixins. It's not particularly pretty, doesn't support method visibility modifiers and, if put to any meaningful purpose, could well make your call stack balloon to Ruby-on-Rails-esque proportions, but it does work.

<?php
abstract class Mix {
    
    protected $_mixMap = array();
    
    public function __construct(){
        
         $this->_mixMap = $this->collectMixins($this);
    }
    
    public function __call($method, $args){
        
        // doesn't pass scope
        //return call_user_func_array(array($className, $method), $args);
        
        // Error: Given object is not an instance of the class this method was declared in
        //$method = new ReflectionMethod($className, $method);
        //return $method->invokeArgs($this, $args);
        
        $payload = $this->buildMixinPayload($this->_mixMap, $method, $args);
        if(!$payload) throw new Exception('Method ' . $method . ' not found');
        
        list($mixinMethod, list($method, $args)) = $payload;
        
        return $this->$mixinMethod($method, $args);
        
    }
    
    protected function collectMixins($class){
        
        static $found = array();
        static $branch = array();
        
        if(empty($branch)) $branch[] = get_class($this);
        $mixins = array();
        
        foreach(array_reverse(get_class_methods($class)) as $method){
            if(preg_match('/^mixin(\w+)$/', $method, $matches)){
                
                $className = $matches[1];
                
                if(in_array($className, $branch))
                    throw new Exception('Circular reference detected ' . implode(' > ', $branch) . ' > ' . $className);
                    
                if(!in_array($className, $found)){
                
                    if(!class_exists($className)) throw new Exception('Class ' . $className . ' not found');
            
                    // populate props from mixin class
                    foreach(get_class_vars($className) as $key => $value){        
                        if(!property_exists($this, $key)) $this->$key = $value;
                    }
                    
                    $found[] = $branch[] = $className;                
                    $mixins[$className] = $this->collectMixins($className);
                }
                
                $branch = array(get_class($this));
            }
        }
        
        return $mixins;
    }
    
    protected function buildMixinPayload($mixins, $method, $args){
        
        foreach($mixins as $className => $parents){
            
            $mixinMethod = 'mixin' . $className;
            
            if(method_exists($className, $method)) return array($mixinMethod, array($method, $args));
            
            if(!empty($parents) && $return = $this->buildMixinPayload($parents, $method, $args)){
                return array($mixinMethod, $return);
            }
        }
        
        return false;
    }
    
}
?>

5 years ago


Model for Mixins pattern:

<?php
interface IMixinsCaller
{
    public function __mixin_get_property($property);

    public function __mixin_set_property($property, $value);

    public function __mixin_call($method, $value);
}

abstract class MixinsCaller implements IMixinsCaller
{
    protected $mixins = array();

    public function __call($name, $arguments)
    {
        if (!empty($this->mixins))
        {
            foreach ($this->mixins as $mixin)
            {
                if (method_exists($mixin, $name))
                {
                    return call_user_func_array(array($mixin, $name), $arguments);
                }
            }
        }
        trigger_error('Non-existent method was called in class '.__CLASS__.': '.$name, E_USER_WARNING);
    }

    public function __mixin_get_property($property)
    {
        if (property_exists($this, $property))
        {
            return $this->$property;
        }
        trigger_error('Non-existent property was get in class '.__CLASS__.': '.$property, E_USER_WARNING);
    }

    public function __mixin_set_property($property, $value)
    {
        if (property_exists($this, $property))
        {
            return $this->$property = $value;
        }
        trigger_error('Non-existent property was set in class '.__CLASS__.': '.$property, E_USER_WARNING);
    }

    public function __mixin_call($method, $value)
    {
        if (method_exists($this, $method))
        {
            return call_user_func_array(array($this, $method), $value);
        }
        trigger_error('Non-existent method was called in class '.__CLASS__.': '.$method, E_USER_WARNING);
    }

    public function AddMixin($mixin)
    {
        $this->mixins[] = $mixin;
    }
}

abstract class Mixin
{
    /** @var IMixinsCaller $parent_object */
    private $parent_object;

    public function __construct(IMixinsCaller $parent_object)
    {
        $this->parent_object = $parent_object;
    }

    public function __get($property)
    {
        return $this->parent_object->__mixin_get_property($property);
    }

    public function __set($property, $value)
    {
        return $this->parent_object->__mixin_set_property($property, $value);
    }

    public function __call($method, $value)
    {
        return $this->parent_object->__mixin_call($method, $value);
    }
}
?>

msg2maciej at aol dot com

5 years ago


PHP supports single class inheritance. My bare idea on accessing protected methods with power of abstracts and sort of "multi-class inheritance SIMULATION":

<?php
error_reporting(E_ALL);

abstract class Base {
    abstract protected function __construct ();
    abstract protected function hello_left ();
    abstract protected function hello_right ();
}

abstract class NotImplemented_Left extends Base {
protected function hello_right () {
echo 'well, wont see that'; }}

abstract class NotImplemented_Right extends Base {
protected function hello_left () {
echo 'well, wont see that'; }}

class Left extends NotImplemented_Left {
protected function __construct () {        # limited visibility, no access from "outside"
echo __CLASS__.'::protected __construct'. "\n"; }
protected function hello_left () {        # limited visibility, no access from "outside"
echo 'protected hello_left in ' . __CLASS__ . "\n"; }}

class Right extends NotImplemented_Right {
protected function __construct () {        # limited visibility, no access from "outside"
echo __CLASS__.'::protected __construct'. "\n"; }
protected function hello_right () {        # limited visibility, no access from "outside"
echo 'protected hello_right in ' . __CLASS__ . "\n"; }
protected function hello_left () {
echo "wont see that, and easy to get rid of it from here\n"; }}

class Center extends Base {
private $left;
private $right;
public function __construct () {
echo 'welcome in ' . __CLASS__ . "\n";
echo 'Center::'; $this->left = new Left;
echo 'Center::'; $this->right = new Right;
echo " oh and\n";
$this->hello_left();
$this->hello_right();
}
public function hello_left () {            # calling class Left
echo __CLASS__.'::'; $this->left->hello_left(); }
public function hello_right () {        # calling class Right
echo __CLASS__.'::'; $this->right->hello_right(); }
}

$c = new Center;
?>

Produces:

welcome in Center
Center::Left::protected __construct
Center::Right::protected __construct
 oh and
Center::protected hello_left in Left
Center::protected hello_right in Right