class Mammal {
	public $eyes = TRUE;
}

class Dog extends Mammal {
	public $fur = TRUE;
}

class Lassie extends Dog {
	public $cute = true;
}


function getDog($type) {
	$class = !class_exists($type) ? 'Dog' : $type;
	
	return new $class;
}



$lassie = getDog('Lassie'); // instance of Lassie
$spot = getDog('Spot'); // instance of Dog