On the ITNewb.com website, there's this new tutorial looking at using the mcrypt libraries in PHP to help protect the data being passed around inside your application.

The ability to encrypt and safeguard data is an essential ability that every serious web developer should have in their arsenal. In this article we'll explore encrypting data with PHP and MCrypt (libmcrypt), storing it in a database, retrieving it from a database and decrypting it for use.

The tutorial walks you through a quick primer on what sort of cryptography the mcyrpt library offers and shows an example of a function (cryptastic!) that takes in the data and a key to return the encrypted value.