On the ITNewb.com site there's a recent tutorial that's been posted by Andrew Johnson about using the mcrypt library in your PHP apps for a simple level of data protection.

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.

He works through what mcrypt is, where it can be downloaded from, a simple usage example and a more in-depth look at what its doing (making an IV, encrypting) and how to store the resulting encrypted information into a MySQL database table.