interface Foo {}
interface Bar extends Foo {}

class Baz implements Foo {}

abstract class A
{
        abstract public function test(Foo $x);
}

class B extends A
{
        public function test (Bar $x) {}
}

function foo(A $foo, Foo $foo) {
    $foo->test($foo); // What should this do?
}

$obj = new B();
foo($obj, new Baz());