This function is slightly older than spl_object_id. Its output is more complex but doesn't actually provide any more information than the newer function. It used to be a lot more complex (without being any more informative) but now it's merely the object's ID number written in hex with a lot of padding to maintain the old format. spl_object_id just gives the ID number as a plain integer.
You're probably better off using spl_object_id, and thinking about migrating if you're already using spl_object_hash; there is a chance this function will be deprecated and subsequently removed in the future.
