PHPBuilder.com has a new article by Hillel Aftel about his method of choice for securing the data that you send over the GET string when moving things around on your site.

In this article I'm going to show you how you can use PHP to encode your data for transit. Most importantly, it will be done in a way that makes the data decodable, and therefore much more usable, by the receiving page.

His encryption method - sending everything in a single string and modifying the scheme each time - uses static pairs of values (of your choosing) to "encode" the outgoing string. It actually consists of three different files: the script itself, a single-use script that makes the third file, an include file to handle the conversion back from the encoded results.