__PHP_Incomplete_Class

PHP serializes its sessions using the built-in serialize and unserialize methods
. serialize of PHP has the ability to serialize PHP objects (aka class instances
) and convert them to string. When you unserialize those strings, It converts 
them back those same classes with those values. Classes who have some private 
properties and want to encode/decode that or do something complex in their 
serialization/deserialization implement the Serializable class and add 
serialize and unserialize methods to the class.

When PHP's unserialize tries to unserialize a class object, but the class name 
isn't declared/required, instead of giving a warning or throwing an Exception, 
it converts it to an object of __PHP_Incomplete_Class.

If you don't want your session objects to convert to __PHP_Incomplete_Class, 
You can do it by either requiring the class files before you invoke 
session_start, or by registering an autoload function.