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PHP : Language Reference : Classes and Objects (PHP 4) : extends

extends

Often you need classes with similar variables and functions to another existing class. In fact, it is good practice to define a generic class which can be used in all your projects and adapt this class for the needs of each of your specific projects. To facilitate this, classes can be extensions of other classes. The extended or derived class has all variables and functions of the base class (this is called 'inheritance' despite the fact that nobody died) and what you add in the extended definition. It is not possible to subtract from a class, that is, to undefine any existing functions or variables. An extended class is always dependent on a single base class, that is, multiple inheritance is not supported. Classes are extended using the keyword 'extends'.

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<?php
class Named_Cart extends Cart {
   var $owner;
 
   function set_owner ($name) {
       $this->owner = $name;
   }
}
?>

This defines a class Named_Cart that has all variables and functions of Cart plus an additional variable $owner and an additional function set_owner(). You create a named cart the usual way and can now set and get the carts owner. You can still use normal cart functions on named carts:

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<?php
$ncart = new Named_Cart;    // Create a named cart
$ncart->set_owner("kris");  // Name that cart
print $ncart->owner;        // print the cart owners name
$ncart->add_item("10", 1);  // (inherited functionality from cart)
?>

This is also called a "parent-child" relationship. You create a class, parent, and use extends to create a new class based on the parent class: the child class. You can even use this new child class and create another class based on this child class.

Note:

Classes must be defined before they are used! If you want the class Named_Cart to extend the class Cart, you will have to define the class Cart first. If you want to create another class called Yellow_named_cart based on the class Named_Cart you have to define Named_Cart first. To make it short: the order in which the classes are defined is important.

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Code Examples / Notes » keyword.extends

"inerte" is my hotmail.com username
[Editor's note: For an alternative to multiple inheritance, see the dynamic binding via object aggregation in the corresponding section of the manual.]
Multiple Inheritance is not supported but it is easy to emulate it:
class multipleInheritance
{
   function callClass($class_to_call)
   {
       return new $class_to_call();
   }
}
class A
{
   function insideA()
   {
       echo "I'm inside A!<br />";
   }
}
class B
{
   function insideB()
   {
       echo "I'm inside B!<br />";
   }
}
class C extends multipleInheritance
{
   function insideC()
   {
       $a = parent::callClass('A');
       $a->insideA();
       $b = parent::callClass('B');
       $b->insideB();
   }
}
$c = new C();
$c->insideC();
---
This will succesfully echo:
I'm inside A!
I'm inside B!

mga
[Editor's note: For an alternative to multiple inheritance, see the dynamic binding via object aggregation in the corresponding section of the manual.]
Multiple Inheritance is not supported but it is easy to emulate it
The example listed above does not show the main advantages of multiple inheritance. I mean, that multiple inheritance is used not for different outputs, but to combain properties ant functionality from different classes.
If you will try to create complex structure based on this example, I am sure, will be big problems regarding member variables values and etc.
In example, each time when you use "inherited" class it is temporarily created!
The main advantage of inheritance is direct incapsulation of member variables and functions. And here are no possibility to use this advantage :(

volte6
When declaring a class that relies upon another file ( because it extends the class defined in that file ), you should ALWAYS require_once() that file at the top.
This applies even when planning on looping through and including everything in the folder. Use require_once() in your loop, and at the top of the file that NEEDS the include.

doedje
tilman's note is not quite true. He states the fact that php gives wrong information about the parent class. But this is not the case. Php does give correct information about the parent class, to prove that I give the following code:
<?php
class a
{
  function showInfo()
  {
     print "I am class " . get_class($this) . ", my parent is " .get_parent_class($this);
  }
}
class b extends a {}
class c extends b {}
$myInstanceB = new b();
$myInstanceB->showInfo();
print "<br />";
$myInstanceC = new c();
$myInstanceC->showInfo();
/*
output:
I am class b, my parent is a
I am class c, my parent is b
*/
?>
The thing is the function showInfo as they were called by my script, are not a member of class a, but are a member of class b and c. They respond accordingly. Showing correct output in the case of get_class and get_parent_class. So don't go blaming php! =]

neelam bhatt
Tilman I think you are little bit confused about get_parent_class() function.
Actually this function return the 'parent class' of the class, object belong to; with which it is called.
In your example one_get_parent_class_name() and two_get_parent_class_name() are similar functions and are located in different classes. Their behaviour will be same if will be called with same object. Just add the following two line at the end. You will get the better understanding.
$two=new two();
$two->two_get_parent_class_name();

tomnezvigin
This may seem obvious, but check this scenario. You have a class folder:
+ class
--classA.php
--classB.php
--classC.php
--mainClass.php
Here... classA, classB, classC all extend the mainClass.
If you try to create a function that automatically includes all of the classes in a folder, normally, they are included alphabetically.
When you try to instantiate classC, for example, you will get an error:
"Cannot inherit from undefined class mainClass"
EVEN IF you instantiate the mainClass before you instantiate all of the other classes.
In other words, make sure your primary class is included before all others.

j237fl
This code fragment does NOT seem to work.  It seems as though, the PHP engine when loading a class dosen't set the member vars FIRST, it seems as though it loads the BasicLife member vars before the Super Class methods.
class BasicLife extends Benefit{
   //MEMBER VAR simply REFERS TO METHOD OF SUPER CLASS
   var $simply = Benefit::get_simply();
   //MEMBER VAR me REFERS TO METHOD OF THIS->CLASS
   var $me = $this->get_me();
   function BasicLife(){//CONSTRUCTOR}
   function get_me(){}
   
}

robertvanderlinden78
the model as in the example of "quinton" is what you want to have. Not the method of "kimh216" with function callClass
when calling
   function C()
   {
       //global $a,$b;
       $this->a = parent::callClass('A');//no effect
       $this->b = parent::callClass('B');//no effect
   }
you will not have 1 data object.If i wanted to add a function to class A and i wanted to call it , I can only do it from Class C .. otherwise if I were to do
$c = new C();
$c->someExtraFunctionThatIsInClassA(); it will give an error stating "Fatal error:  Call to undefined function:  functionName() in f:\ibserver\www\meuk\classvbs\test.php.php on line 34"
------------
If i add a new function in Person class of the first example of quinton:
 function boe(){
 echo "some text";
 }
then i can call it by :
$family->members[0]->boe();
since the members array has Personobjects in it. Now you have 1 Object that can access every function as a whole.
regards rml

msquared
Multiple inheritence is often more trouble than it's worth.  For example, you have a class foo that inherits from both class bar and class baz.  Classes bar and baz both have a fubar() method.  When you create a foo object and call its fubar() method, which fubar() method is called: bar's or baz's?
It seems to me that using aggregate to glue one class's methods and data to another object is a bit like Ruby's fixins, but I could be wrong...
[[Editor's note:
The aggregate_* functions have been dropped, as of PHP 5
-S
]]

jo
Multiple inheritance, if designed properly into the language, most definitely is worth the trouble. Solutions exist for all associated problems (including the name clash issue mentioned above - identify a function by adding the name of the class where is was first defined, and all ambiguities vanish; make the class identification optional if nonambiguous, and it's even backwards-compatible).
The rub is that it's difficult to get a language design that's simple to use, efficiently implementable, and applicable to a dynamically-typed language like PHP.

griffon9
Just to clarify something about inheritance. The following code :
class a
{   function call()
    {  $this->toto();
    }
   
    function toto()
    { echo('Toto of A');
    }
}
 
class b extends a
{  function toto()
   { echo('Toto of B');
   }
}
$b=new b;
$b->call();
...will correctly display "toto of B" (that is, the function declared in the parent is correctly calling the redefined function in the child)

bpotier
Just one thing that may seem obvious but not mentionned in this page is that you need to include/require the file containing the parent class or else you'll get an error:
<?php
require(dirname(__FILE__).'/'.'myParent.php');
...
myChild extends myParent {
...
}
...
?>

mazsolt
Just a simple example about inheritance:
class a1{
 var $a=10;
 function a1($a){
    $this->a=$a;
 }
}
class a2 extends a1{
 var $x=11;
 function a2($x,$y){
    $this->x=$x;
    parent::a1($y); // or a1::a1($y) or $this->a1($y)
 }
}
class a3 extends a2{
 var $q=999;
}
$x=new a3(99,9);
echo $x->a,"
",$x->x,"
",$x->q;
The output will be:
9
99
999

calimero
Just a quick note to make things more clear : while multiple inheritance is not allowed, several levels of single inheritance  ARE ALLOWED indeed. Just test this example :
<?
class A {
var $name='A';
function disp() {
echo $this->name;
}
}
class B extends A {
var $name='B';
}
class C extends B {
var $name='C';
}
$truc = new C() ;
$truc->disp(); // Will output C
?>
This is especially important to keep in mind while building a huge object hierarchy. for example :
+GenericObject
->+ Person
->->Employee
->+Computer
->->+WorkStation
->->-> PPC
->->-> Intel
->->+Server
->->->LDAPServer
->->->IntranetWebServer
.. and so on. Multiple level hierarchy relationship are possible in a tree-like structure (each child has one and only one parent, except for the root object).

alan hogan
Just a note:  It is possible to have a class inherit from multiple other classes, but only in a one-at-a-time linear hierarchy.
So this works, and C gets A and B functions:
<?php
class A {
 public function af() { print 'a';}
 public function bark() {print ' arf!';}
}
class B extends A {
 public function bf() { print 'b';}
}
class C extends B {
 public function cf() { print 'c';}
 public function bark() {print ' ahem...'; parent::bark();}
}
$c = new C;
$c->af(); $c->bf(); $c->cf();
print "<br />";
$c->bark();
/**results:**/
//abc
//ahem... arf!
?>
This does NOT work:
<?php
class A {
 public function af() { print 'a';}
 public function bark() {print ' arf!';}
}
class B {
 public function bf() { print 'b';}
}
class C extends B, A /*illegal*/ {
 public function cf() { print 'c';}
 public function bark() {print ' ahem...'; parent::bark();}
}
$c = new C;
$c->af(); $c->bf(); $c->cf();
print "<br />";
$c->bark();
//Parse Error
?>

efredin
It is possible to override a method innherited from a parent class by simply re-defining the method (for those of us who enjoy using abstract classes).
<?
class A
{
var $foo;
function A()
{
$this->foo = "asdf";
}

function bar()
{
echo $this->foo." : Running in A";
}
}
class B extends A
{
function bar()
{
echo $this->foo." : Running in B";
}
}
$myClass = new B;
$myClass->bar();
?>

novaki
inheritance from multiple parent classes:
// won't work
class myChildClass extends pClass1, pClass2 {
}
PHP uses aggregation for inheriting from multiple parents, which I think is a bit more complex. See the aggregate function.
(my personal oppinion is that the first method of multiple inheritance should also be imlemented, but I got the answer that it won't be fixed)

edward_nl
If you are using a child-class. Remember to call the constructor of the parent class aswell before you start using it. Otherwise you might get different results then you expected. It is stated in this document, but I got confused by the given example. So, here is my example:
<?php
error_reporting(E_ALL);
class test {
 var $var;
 function test() {
   $this->var = 3;
 }
}
class testing extends test {
  function testing() {
    parent::test();
  }
  function My_test() {
    return $this->var;
  }
}
$p = new testing();
echo $p->My_test();
// Returns 3

php_at_undeen_dot_com
if the class B that extends class A does not have a constuctor function (i.e. a function named B), then the constructor function of A will be used instead, you don't need to make a constructor in B just to call the constructor of A.
For example:
class A
{
 function A()
   {
     echo "HEY! I'm A!\n";
   }
}
class B extends A
{
}
$b = new B();
produces the output:
HEY! I'm A!

roy yong
I'm not sure if i'm outdated but I think the current PHP version still does not support extension of methods (something that stands between polymorphism and overriding). Hopefully, this could be included in the next release version of PHP!

dave fancella
I recommend against using the examples here given for multiple inheritance.
The essential problem is having readable code.  If you want to be able to read your code in a few months, or if you want others to be able to read your code, you're better off using member variables to store class instances than using hacked on multiple inheritance.
One of the big advantages to multiple inheritance in a language that properly supports it like C++ is that you can see, in the class declaration, what it inherits from.  In the example where you extend some class called MultipleInheritance, that only obfuscates your class's ancestry.
So, use inheritance, it's good.  But it's not the be all end all of solutions.  If it were, we wouldn't have templates and various other things in C++.  So don't depend on it.  Instead of asking yourself "How do I use inheritance to solve this problem?" you should ask yourself "Is inheritance the right tool to solve this problem?"  If the language doesn't support multiple inheritance and you're trying to use it anyway, then you have certainly chosen the wrong tool to solve your problem.

kimh216
I made some change...
If you don't want to call parent::callClass('A') on the every function which have to call it's parent's method.
//multipleInheritance
class multipleInheritance
{
   function callClass($class_to_call)
   {
       return new $class_to_call();
   }
}
class A
{
   function insideA()
   {
       echo "I'm inside A!<br />";
   }
}
class B
{
   function insideB()
   {
       echo "I'm inside B!<br />";
   }
}
class C extends multipleInheritance
{
   var $a;
   var $b;
   function C()
   {
       //global $a,$b;
       $this->a = parent::callClass('A');//no effect
       $this->b = parent::callClass('B');//no effect
   }
   function insideA()
   {
      // global $a,$b;
       $this->a->insideA();
   }
   function insideB()
   {
       //global $a,$b;
       $this->b->insideB();
   }
   function insideC()
   {
       echo("I'm inside C!
");
   }
}
$c = new C();
$c->insideA();
$c->insideB();
$c->insideC();

tilman dot schroeder
Hi...
as alan hogan pointed out it is possible to inherit a class from multiple other classes but notice that php does not always give you the right name of the parent class if you are deriving from multiple classes (tested on php 4.4.0):

<?php
header("Content-Type: text/plain");
class three {
function three() {
echo("constructor of three\n");
} // end function three
} // end class three

class two extends three {
function two() {
echo("constructor of two\n");
} // end function two
function two_get_parent_class_name() {
echo("i am two and my parent is: ".get_parent_class($this)."\n");
} // end one_function get_parent_class_name()
} // end class two
class one extends two {
function one() {
echo("constructor of one\n");
} // end function one
function one_get_parent_class_name() {
echo("i am one and my parent is: ".get_parent_class($this)."\n");
} // end one_function get_parent_class_name()
} // end class one
$one=new one();
$one->two();
$one->three();
$one->one_get_parent_class_name();
$one->two_get_parent_class_name();
/*
* will print out:
*
* constructor of one
* constructor of two
* constructor of three
* i am one and my parent is: two
* i am two and my parent is: two // should print out "my parent is: three"
*/
?>
So be careful when dealing with parents within classes ...
However, if you are using parent:: it will give you access to BOTH parent classes:
<?php
header("Content-Type: text/plain");
class three {
function three() {
echo("three\n");
}
}
class two extends three {
function two() {
echo("two\n");
}
}
class one extends two {
function one() {
echo("one\n");
parent::two();
parent::three();
}
}
one::one();
/*
* will print out:
* one
* two
* three
*/
?>

bash i.
Here is a simple idea that I use when I need my abstract classes (the inherited classes) implemented before my functional classes.
<?php

$_CLASSES = array_merge (
glob ("classes/*/*.abstract.php"),
glob ("classes/*/*.class.php")
);

foreach ($_CLASSES AS $_CLASS) {
require ($_CLASS);
}

?>

schultz
class a {
 function samename(){
   echo 'a';
} }
class b extends a{
 function samename(){
   echo 'b';
 }
 function b(){
   a::samename();
   b::samename();
} }
$test_obj = new b();
Quiet confusing though, as this prints out 'ab'.
No need to create a new instance of a,
therefor both methods still exists with same name.

quinton
a nice example using extends and multiple classes  and constructors.
<?
class CoreObject {
 var $name;
 
 function CoreObject($name){
   $this->_constructor($name);
 }
 
 function _constructor($name){
   $this->name = $name;
 }
 function show(){
   printf("%s::%s\n", $this->get_class(), $this->name);
 }
 
 function get_class(){
  return get_class($this);
 }
}
class Container extends CoreObject{
var $members;
function Container($name){
  $this->_constructor($name);
}

function &add(&$ref){
  $this->members[] = $ref;
  return ($ref);
}

 function show(){
  parent::show();
  foreach($this->members as $item){
    $item->show();
  }
}
function apply(){
}
}
class Person extends CoreObject{
 function Person($name){
   $this->_constructor($name);
 }
}
class Family extends Container {
var $members;
function Family($name){
  $this->_constructor($name);
}
}
echo "<pre>\n";
$family = new Family('my family');
$family->add(new Person('father'));
$family->add(new Person('mother'));
$family->add(new Person('girl'));
$family->add(new Person('boy'));
$family->show();
print_r($family);
?>

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