PHP Unit Testing
Chris Shiflett has posted an interesting entry on Unit Testing in PHP, together with a PDF slideshow (Power PHP Testing - PDF link!).
The slideshow is actually quite interesting, because it goes through all the unit testing packages (like PHP-Unit, SimpleTest, etc) and reviews them. It also gives sample code for each package, which is a nice addition, especially if you're looking for a simple package.
I haven't done any unit testing myself yet, because I've never had to, and I wouldn't know how to do it properly, but I should probably start doing it soon, especially with my commercial projects (I do use a similar form of unit testing, except it isn't really unit testing).