On SitePoint there's a new tutorial that introduces you to some of the basics of unit testing your PHP application with the help of the popular testing tool PHPUnit. (Disclaimer: I am the author of this article).
It’s a familiar situation: you’ve been developing on an application for hours and you feel like you’ve been going round in circles. You fix one bug and another pops up. [...] You’ve been there, right? You’ve had these same frustrations with all your applications, and have sat back and thought that there has to be a better way. Well, you’re in luck! There is, and it’s not as difficult as you might think it.
The article starts by debunking a few of the myths about unit testing like the time it takes and why it's better than just "clicking around". It shows the installation of the tool (via the PEAR installer) and how to write your first test cases based on a sample class.