session question: session.auto_start vs. session_register.

I am working on an open source e-commerce package and have hit a wall with sessions. If I have session.auto_start turned on, I get the following error message: Fatal error: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition shoppingcart of the object you are trying to operate on was loaded _before_ the session was started in <path to my file> on line 12 If I turn session.auto_start off, the error disappears. So, the error message tells me that I can't use the class unless I've defined it before the session gets started. However, session.auto_start (as far as I know) starts a session immediately, before even waiting for a script to be fully parsed & executed. So, the two seem mutually exclusive. (but then the usefullness of session.auto_start would seem extremely limited) Is there a way I think there's some glaring errors in my understanding here. Can someone help me fill in the holes? --kurt