php incomplete class

1PHP serializes its sessions using the built-in serialize and unserialize methods
2. serialize of PHP has the ability to serialize PHP objects (aka class instances
3) and convert them to string. When you unserialize those strings, It converts 
4them back those same classes with those values. Classes who have some private 
5properties and want to encode/decode that or do something complex in their 
6serialization/deserialization implement the Serializable class and add 
7serialize and unserialize methods to the class.
8
9When PHP's unserialize tries to unserialize a class object, but the class name 
10isn't declared/required, instead of giving a warning or throwing an Exception, 
11it converts it to an object of __PHP_Incomplete_Class.
12
13If you don't want your session objects to convert to __PHP_Incomplete_Class, 
14You can do it by either requiring the class files before you invoke 
15session_start, or by registering an autoload function.