unit tests and object-inheritance

Hey! I have a question concerning "unit tests" and object-inheritance. For example:

I have a class A which extends class B. Let's assume the only difference of the two classes is the add method . In class B this add method is slightly extended. Now I want to write a unit test for the add function of class B but because of the parent::add call I have a dependency to the parent class A. In this case I can't mock the add method of the parent class so the resulting test will be a integration test but if I want it to be a unit test? I don't want the the test for method add in class B fails because of the parent method in class A. In this case only the unit-test of the parent method should fail.

class B exends A 
{
    public function add()
    {
        parent::add();
        //do some additional stuff  
    }
    ....
}

class A 
{
    public function add(){...}
    ....
}

Surely I could use object-aggregation and pass my parent object to the child contructor and therefore I would be able to mock the parent method add, but is this the best approach? I would rarely use object-inheritance anymore.

class B {
    protected $a;

    public function __contruct(A $a)
     {
        $this->a = $a;
    }

    public function add()
    {
        $this->a->add();
        //do some additional stuff  
    }
    ....
}

class A {
    public function add(){...}
    ....
}

I would be very grateful for your opinions. Thanks!

Karl