At the recent International PHP Conference held in Frankfurt, Germany I was able to pick up a copy of Sebastian Bergmann's "PHPUnit Pocket Guide" a slim pocket guide published by O’Reilly which forms the most complete documentation for PHPUnit.

What I found enjoyable, was the mixture between real-world examples, Agile development methodologies and program documentation. I have never used PHPUnit style of unit testing before and while I am familiar with the concepts of Agile and Extreme programming, I would have appreciated a bit more Theory. Of course, this is a pocket guide, not "Unit Testing Unleashed" and links are provided to various sources of information on the topic, so there is plenty of information for me to look at.

He also mentions two other sections in the book - PHPUnit with Phing and with PHP4. And, of course, Aaron reminds us that the contents of the book are also a totally free download from the PHPUnit site too...