php 向上轉型

<?php
interface  animal{
	public function sound();
}


class duck implements animal{
	public function sound(){
		echo "duck sound";
	}
}

class dog implements animal{
	public function sound(){
		echo "dog sound";
	}
}



class foo{
	function makeSound(animal $obj){
	$obj->sound();
}	
}

$bar  = new foo();
$bar->makeSound(new dog());
$bar->makeSound(new duck());
?>