/**
 * @author you
 */
abstract class A
{
    abstract public function a (); //this is a contract
}
/**
 * @author someone else
 */
abstract class B extends A
{
    abstract public function a ($t = false);
}
/**
 * @author someone else
 */
class ConcreteA extends A
{
    public function a()
    {
        //do something
    }
}

/**
 * @author someone else
 */
class ConcreteB extends B
{
    public function a($t = false)
    {
        //do something
    }
}

/**
 * For handling A's
 * @author you
 */
class AHandler
{
    public function doA(A $object)
    {
        $object->a();
    }
}


//someone elses code...
$obj1 = new ConcreteB();
$obj2 = new ConcreteA();

//your code
$handler = new AHandler();
$handler->doA($obj2); //works fine
$handler->doA($obj1); //typehint works fine, so probably something like "call to undefined method a"