Built-in Interface to rewrite empty() ?

Hi, I think I once knew a PHP5 interface, which forces a class to implement a function empty(). If an object $myObject implements this interface, it should be possible to call empty($myObject) as it seems to work with the interface "Countable", which does the same for count(). The problem is I didn't find such an interface in any doc I found so far. Does anyone of you know such an interface, or did it ever exist? thanks, Norbert