If you get such a error That means class not defined before starting session.
If classes stored in a session you must define your class before the call to session start().
If you have in configuration session auto_start = 1, you will have to disable that so you can define your classes before starting the session.
Another easiest workaround is to define __autoload() function before session start(). This way the appropriate classes are loaded when needed and it will solve the error. also you do not need to make any changes in configuration file