Question

Class C extends B. And class B extends either A1 or A2, depending on an argument of C’s constructor. How to define class A, B and C?

Answer 1

Like deceze already points out, this is not possible with inheritance.

You could use PECL’s runkit_class_adopt to convert a base class to an inherited class, add ancestral methods when appropriate but runkit is something you dont want in your production code.

The clean OO approach to this would be to use a Bridge pattern to give the object the implementation of the A class at runtime and thereby

decoupling an abstraction from its implementation so that the two can vary independently.

How it works

First define an interface every "parent" of B and B itself has to implement. The interface should contain all the methods you want B to"inherit". I am putting these terms in quotes, because technically they are neither parents, nor inheriting anything.

interface MyImplementation
{
    public function doSomething();
    // more methods …
}

Then define classes A1 and A2 implementing this interface and add the methods demanded by the interface.

class A1 implements MyImplementation
{
    public function doSomething()
    {
        return 'A1';
    }
    // more methods …
}
class A2 implements MyImplementation
{
    public function doSomething()
    {
        return 'A2';
    }
    // more methods …
}

Next create your class B and make it require one of the implementing classes on initialization.

abstract class B implements MyImplementation
{
    protected $_implementation;
    // more properties ...

    public function __construct(MyImplementation $implementationObj)
    {
        $this->_implementation = $implementationObj;
    }
    public function doSomething()
    {
        return $this->_implementation->doSomething();
    }
    // more methods ...
}

As you can see, any calls to the methods required by our interface are delegated to the aggregated "parent" object, so depending on what you passed into B, you will get different results.

Now to define C we just make it accept the class name of the implementation class and instantiate it within C to pass it to the parent, e.g. B.

class C extends B {
    public function __construct($bMethodImplementation) {
        parent::__construct(new $bMethodImplementation);
    }
}

and then you can do

$c = new C('A1');
echo $c->doSomething(); // echoes A1
$c = new C('A2');
echo $c->doSomething(); // echoes A2