Ciao.
Ho trovato questo :
I've just come accross something interesting relating to storing PHP5 objects in a session. If you don't provide an __autoload(), then you MUST load the class definition before calling session_start(). I guess that when you call session_start(), any objects in the session are unserialized then and there and placed into $_SESSION. If you don't provide the class definition before calling session_start(), your object will get the class __PHP_Incomplete_Class, and you won't be able to use it for anything.
Penso ti possa essere utile.