Chris Hartjes (aka the "Grumpy Programmer") has written a second book that aims to teach you even more about writing testable applications. Where his first book taught the basics of making apps testable, this new book - The Grumpy Programmer's PHPUnit Cookbook provides code examples showing how to solve some of the common problems you'll come across when testing your applications.

You know you need something better, but time just doesn't seem to be on your side. Making things "right" is costly and you need to deliver working code NOW. Tests would be great but there are real deadlines to meet. You can't stop development and churn away for hours just to add tests around what you know already works. [...] No longer would you dread the bug reports. You'd happily make changes knowing that your safety net is there. You'd try out new features guiding yourself with tests.

The book covers some more advanced topics than you might have seen in the previous book including data providers, test doubles, faking test data (or using external sources) and writing tests for APIs, databases and exceptions. If this sounds interesting to you, you can pick up your own copy of the book on his site (or try out the sample first).