PHP5 has a great feature for us lazy OOP people. With __autoload() you can skip over some of the hell of including every class file. But wait. Don't use __autoload()! Instead, so that your script plays nice with others you should use spl_autoload_register('my_overload_function'); This allows multiple autoload functions so that yours doesn't hog __autoload.
Now, onto what tripped me up for a few minutes. I placed a class in the session and then registered the autoload function in my cart include file. Instead of having everything magically work, I got this error: