Can I extend a class using more than 1 class in PHP?


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If I have several classes with functions that I need but want to store separately for organisation, can I extend a class to have both?

i.e. class a extends b extends c

edit: I know how to extend classes one at a time, but I'm looking for a method to instantly extend a class using multiple base classes - AFAIK you can't do this in PHP but there should be ways around it without resorting to class c extends b, class b extends a



All Answers
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    Answering your edit :

    If you really want to fake multiple inheritance, you can use the magic function __call().

    This is ugly though it works from class A user's point of view :

    class B {
        public function method_from_b($s) {
            echo $s;
        }
    }
    
    class C {
        public function method_from_c($s) {
            echo $s;
        }
    }
    
    class A extends B
    {
      private $c;
    
      public function __construct()
      {
        $this->c = new C;
      }
    
      // fake "extends C" using magic function
      public function __call($method, $args)
      {
        $this->c->$method($args[0]);
      }
    }
    
    
    $a = new A;
    $a->method_from_b("abc");
    $a->method_from_c("def");
    

    Prints "abcdef"


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    You cannot have a class that extends two base classes. You could not have.

    // this is NOT allowed (for all you google speeders)
    Matron extends Nurse, HumanEntity
    

    You could however have a hierarchy as follows...

    Matron extends Nurse    
    Consultant extends Doctor
    
    Nurse extends HumanEntity
    Doctor extends HumanEntity
    
    HumanEntity extends DatabaseTable
    DatabaseTable extends AbstractTable
    

    and so on.


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    You could use traits, which, hopefully, will be available from PHP 5.4.

    Traits is a mechanism for code reuse in single inheritance languages such as PHP. A Trait is intended to reduce some limitations of single inheritance by enabling a developer to reuse sets of methods freely in several independent classes living in different class hierarchies. The semantics of the combination of Traits and classes is defined in a way, which reduces complexity and avoids the typical problems associated with multiple inheritance and Mixins.

    They are recognized for their potential in supporting better composition and reuse, hence their integration in newer versions of languages such as Perl 6, Squeak, Scala, Slate and Fortress. Traits have also been ported to Java and C#.

    More information: https://wiki.php.net/rfc/traits


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    Classes are not meant to be just collections of methods. A class is supposed to represent an abstract concept, with both state (fields) and behaviour (methods) which changes the state. Using inheritance just to get some desired behaviour sounds like bad OO design, and exactly the reason why many languages disallow multiple inheritance: in order to prevent "spaghetti inheritance", i.e. extending 3 classes because each has a method you need, and ending up with a class that inherits 100 method and 20 fields, yet only ever uses 5 of them.


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    There are plans for adding mix-ins soon, I believe.

    But until then, go with the accepted answer. You can abstract that out a bit to make an "extendable" class:

    class Extendable{
      private $extender=array();
    
      public function addExtender(Extender $obj){
        $this->extenders[] = $obj;
        $obj->setExtendee($this);
      }
    
      public function __call($name, $params){
        foreach($this->extenders as $extender){
           //do reflection to see if extender has this method with this argument count
           if (method_exists($extender, $name)){
              return call_user_func_array(array($extender, $name), $params);
           }
        }
      }
    }
    
    
    $foo = new Extendable();
    $foo->addExtender(new OtherClass());
    $foo->other_class_method();
    

    Note that in this model "OtherClass" gets to 'know' about $foo. OtherClass needs to have a public function called "setExtendee" to set up this relationship. Then, if it's methods are invoked from $foo, it can access $foo internally. It will not, however, get access to any private/protected methods/variables like a real extended class would.


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    Currently accepted answer by @Franck will work but it is not in fact multiple inheritance but a child instance of class defined out of scope, also there is the __call() shorthand - consider using just $this->childInstance->method(args) anywhere you need ExternalClass class method in "extended" class.

    Exact answer

    No you can't, respectively, not really, as manual of extends keyword says:

    An extended class is always dependent on a single base class, that is, multiple inheritance is not supported.

    Real answer

    However as @adam suggested correctly this does NOT forbids you to use multiple hierarchal inheritance.

    You CAN extend one class, with another and another with another and so on...

    So pretty simple example on this would be:

    class firstInheritance{}
    class secondInheritance extends firstInheritance{}
    class someFinalClass extends secondInheritance{}
    //...and so on...
    

    Important note

    As you might have noticed, you can only do multiple(2+) intehritance by hierarchy if you have control over all classes included in the process - that means, you can't apply this solution e.g. with built-in classes or with classes you simply can't edit - if you want to do that, you are left with the @Franck solution - child instances.

    ...And finally example with some output:

    class A{
      function a_hi(){
        echo "I am a of A".PHP_EOL."<br>".PHP_EOL;  
      }
    }
    
    class B extends A{
      function b_hi(){
        echo "I am b of B".PHP_EOL."<br>".PHP_EOL;  
      }
    }
    
    class C extends B{
      function c_hi(){
        echo "I am c of C".PHP_EOL."<br>".PHP_EOL;  
      }
    }
    
    $myTestInstance = new C();
    
    $myTestInstance->a_hi();
    $myTestInstance->b_hi();
    $myTestInstance->c_hi();
    

    Which outputs

    I am a of A 
    I am b of B 
    I am c of C 
    

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    Use traits as base classes. Then use them in a parent class. Extend it .

    trait business{
      function sell(){
    
      }
    
      function buy(){
    
      }
    
      function collectMoney(){
      }
    
    }
    
    trait human{
    
       function think(){
    
       }
    
       function speak(){
    
       }
    
    }
    
    class BusinessPerson{
      use business;
      use human;
      // If you have more traits bring more
    }
    
    
    class BusinessWoman extends BusinessPerson{
    
       function getPregnant(){
    
       }
    
    }
    
    
    $bw = new BusinessWoman();
    $bw ->speak();
    $bw->getPregnant();
    

    See now business woman logically inherited business and human both;


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    <?php
    // what if we want to extend more than one class?
    
    abstract class ExtensionBridge
    {
        // array containing all the extended classes
        private $_exts = array();
        public $_this;
    
        function __construct() {$_this = $this;}
    
        public function addExt($object)
        {
            $this->_exts[]=$object;
        }
    
        public function __get($varname)
        {
            foreach($this->_exts as $ext)
            {
                if(property_exists($ext,$varname))
                return $ext->$varname;
            }
        }
    
        public function __call($method,$args)
        {
            foreach($this->_exts as $ext)
            {
                if(method_exists($ext,$method))
                return call_user_method_array($method,$ext,$args);
            }
            throw new Exception("This Method {$method} doesn't exists");
        }
    
    
    }
    
    class Ext1
    {
        private $name="";
        private $id="";
        public function setID($id){$this->id = $id;}
        public function setName($name){$this->name = $name;}
        public function getID(){return $this->id;}
        public function getName(){return $this->name;}
    }
    
    class Ext2
    {
        private $address="";
        private $country="";
        public function setAddress($address){$this->address = $address;}
        public function setCountry($country){$this->country = $country;}
        public function getAddress(){return $this->address;}
        public function getCountry(){return $this->country;}
    }
    
    class Extender extends ExtensionBridge
    {
        function __construct()
        {
            parent::addExt(new Ext1());
            parent::addExt(new Ext2());
        }
    
        public function __toString()
        {
            return $this->getName().', from: '.$this->getCountry();
        }
    }
    
    $o = new Extender();
    $o->setName("Mahdi");
    $o->setCountry("Al-Ahwaz");
    echo $o;
    ?>
    

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    I have read several articles discouraging inheritance in projects (as opposed to libraries/frameworks), and encouraging to program agaisnt interfaces, no against implementations.
    They also advocate OO by composition: if you need the functions in class a and b, make c having members/fields of this type:

    class C
    {
        private $a, $b;
    
        public function __construct($x, $y)
        {
            $this->a = new A(42, $x);
            $this->b = new B($y);
        }
    
        protected function DoSomething()
        {
            $this->a->Act();
            $this->b->Do();
        }
    }
    

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    Multiple inheritance seems to work at the interface level. I made a test on php 5.6.1.

    Here is a working code:

    <?php
    
    
    interface Animal
    {
        public function sayHello();
    }
    
    
    interface HairyThing
    {
        public function plush();
    }
    
    interface Dog extends Animal, HairyThing
    {
        public function bark();
    }
    
    
    class Puppy implements Dog
    {
        public function bark()
        {
            echo "ouaf";
        }
    
        public function sayHello()
        {
            echo "hello";
        }
    
        public function plush()
        {
            echo "plush";
        }
    
    
    }
    
    
    echo PHP_VERSION; // 5.6.1
    $o = new Puppy();
    $o->bark();
    $o->plush();
    $o->sayHello(); // displays: 5.6.16ouafplushhello
    

    I didn't think that was possible, but I stumbled upon in the SwiftMailer source code, in the Swift_Transport_IoBuffer class, which has the following definition:

    interface Swift_Transport_IoBuffer extends Swift_InputByteStream, Swift_OutputByteStream
    

    I didn't play with it yet, but I thought it might be interesting to share.


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    I just solved my "multiple inheritance" problem with:

    class Session {
        public $username;
    }
    
    class MyServiceResponsetype {
        protected $only_avaliable_in_response;
    }
    
    class SessionResponse extends MyServiceResponsetype {
        /** has shared $only_avaliable_in_response */
    
        public $session;
    
        public function __construct(Session $session) {
          $this->session = $session;
        }
    
    }
    

    This way I have the power to manipulate session inside a SessionResponse which extends MyServiceResponsetype still being able to handle Session by itself.


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    You are able to do that using Traits in PHP which announced as of PHP 5.4

    Here is a quick tutorial for you, http://culttt.com/2014/06/25/php-traits/


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    PHP does not yet support multiple class inheritance, it does however support multiple interface inheritance.

    See http://www.hudzilla.org/php/6_17_0.php for some examples.


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    PHP does not allow multiple inheritance, but you can do with implementing multiple interfaces. If the implementation is "heavy", provide skeletal implementation for each interface in a seperate class. Then, you can delegate all interface class to these skeletal implementations via object containment.


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    Not knowing exactly what you're trying to achieve, I would suggest looking into the possibility of redesigning you application to use composition rather than inheritance in this case.


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    Always good idea is to make parent class, with functions ... i.e. add this all functionality to parent.

    And "move" all classes that use this hierarchically down. I need - rewrite functions, which are specific.


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    If you want to check if a function is public see this topic : https://stackoverflow.com/a/4160928/2226755

    And use call_user_func_array(...) method for many or not arguments.

    Like this :

    class B {
        public function method_from_b($s) {
            echo $s;
        }
    }
    
    class C {
        public function method_from_c($l, $l1, $l2) {
            echo $l.$l1.$l2;
        }
    }
    
    class A extends B {
        private $c;
    
        public function __construct() {
            $this->c = new C;
        }
    
        public function __call($method, $args) {
            if (method_exists($this->c, $method)) {
                $reflection = new ReflectionMethod($this->c, $method);
                if (!$reflection->isPublic()) {
                    throw new RuntimeException("Call to not public method ".get_class($this)."::$method()");
                }
    
                return call_user_func_array(array($this->c, $method), $args);
            } else {
                throw new RuntimeException("Call to undefined method ".get_class($this)."::$method()");
            }
        }
    }
    
    
    $a = new A;
    $a->method_from_b("abc");
    $a->method_from_c("d", "e", "f");
    

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    One of the problems of PHP as a programming language is the fact that you can only have single inheritance. This means a class can only inherit from one other class.

    However, a lot of the time it would be beneficial to inherit from multiple classes. For example, it might be desirable to inherit methods from a couple of different classes in order to prevent code duplication.

    This problem can lead to class that has a long family history of inheritance which often does not make sense.

    In PHP 5.4 a new feature of the language was added known as Traits. A Trait is kind of like a Mixin in that it allows you to mix Trait classes into an existing class. This means you can reduce code duplication and get the benefits whilst avoiding the problems of multiple inheritance.

    Traits


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    This is not a real answer, we have duplicate class

    ...but it works :)

    class A {
        //do some things
    }
    
    class B {
        //do some things
    }
    

    class copy_B is copy all class B

    class copy_B extends A {
    
        //do some things (copy class B)
    }
    
    class A_B extends copy_B{}
    

    now

    class C extends A{}
    class C extends B{}
    class C extends A_B{}  //extends A & B
    

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    class A extends B {}

    class B extends C {}

    Then A has extended both B and C