<?php

interface AnInterface
{
        public function method();
}

class AClass implements AnInterface
{
        public function method()
        {
                echo __METHOD__;
        }
}

abstract class AnAbstractClass
{
        abstract public function method( AnInterface $Object );
}

class ConcreteClass extends AnAbstractClass
{
        public function method( AClass $Object )
        {
                $Object->method();
        }
}

$Object1 = new ConcreteClass();
$Object2 = new AClass();

$Object1->method( $Object2 );

assert( is_a($Object, "AClass") );

<?php
class BClass implements AnInterface { }

function moo(AnAbstractClass $abstract)
{
    $b = new BClass();
    $abstract->method($b);
}