What is Serialized PHP?
Serialized PHP is a data encoding format for PHP. It is easy to store and transmit and is a convenient format for PHP developers to work with. It’s built-in support in the PHP programming language makes it a good choice for PHP application developers. (from Yahoo)

PHP supports this from version 3.0.5 onwards. To serialize a datatype in PHP, all you need is serialize(), and to retrieve the datatype back into PHP you use unserialize().

What serialize does is to convert the PHP data-type into a byte-stream representation, which can be transmitted or stored. The datatypes which can be converted are: boolean, integer, float, string, array or object. When serialized data is to be retrieved into PHP, running that data through unserialize should get back your initial datatype.
Here’s a link to Yahoo showing you how to use Serialized PHP for Web Applications: Using Serialized PHP with Yahoo! Web Services