On O'Reilly's PHPDev Center there's a new article about Calculating Entropy for Data Miners. Paul Meagher writes about the movement of data from low organization to higher organization (entropy).

This article discusses the concepts of joint and conditional entropy through developing the corresponding classes--JointEntropy.php, ConditionalEntropy--and illustrating how these classes allow web database programmers quick access to an entropic analysis of the relationship between two or more database columns considered at a time.

He gives plenty of code and logic examples, showing you how you can use these handy chunks of code to measure the entropy between your different data items. It's a more high-level concept than most of the tutorials, but can be immensely useful when you really need it...