Insight into Object Serialization and Loading

Quick question to the powers that be. With the current implementation of objects and such, how flexible is unserialize and serialize. I am currently building a large object oriented web application, and I have a pile of moduls and definition files stored to disk as serialized objects. Just wondering how much flexibility I have with changing class definitions (adding/removing member variables). If I add 5 variables to the class definitions ....will my app fail terrible when unserializing? How about if I get rid of some variables from the class definition. Or, when unserializing, does PHP just match up whatever variables it can, and then it doesn't worry about the rest? Insight appreciated. -Brian Tanner