[php] PHP_Incomplete_Class

You've saved an object instance of YourClassName into session. On a subsequent retrieval of this object from session, you discover it's now a __PHP_Incomplete_Class object!

your_object = __PHP_Incomplete_Class Object
(
  [__PHP_Incomplete_Class_Name] => YourClassName
  [id] => 123
  [name] => Fred
)

Check where you're doing the session_start. Chances are you did not include YourClassName (plus any required files of its inheritance chain, if applicable) before starting the session.