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phpoopkohanakohana-3kohana-orm

What is the purpose of empty abstract classes in Kohana 3?

I haven't used abstract classes much in practice, though I understand what they are : a way to dictate to subclasses which methods must be implemented.

I just put a Kohana project through Doxygen and can see the class hierarchy in its entirety. I see that, at the top of the chain we have a factory:

abstract class Kohana_Model { 

    public static function factory($name){
        // Add the model prefix
        $class = 'Model_'.$name;
        return new $class;
    }

}

Inherited directly below that, we have an empty abstract class:

abstract class Model extends Kohana_Model {}

... And below that, there are three inherited classes: Kohana_Model_Database, Kohana_ORM, and Model_Foobar.

Would someone please explain the programming reasoning for this - what is the purpose of having an empty abstract class this high up in the chain? (and, at all?)

Why not have Kohana_Model_Database, Kohana_ORM, and Model_Foobar inherit directly from Kohana_Model, when there is (apparently?) no other branching or inheritance going on between Model and Kohana_Model?

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