I just put down my copy of Hands-on Testing with PHPUnit How-to by Michael Lively, published through Packt Publishing.
Overall, this is a pretty good book, and covers all of the topics one would expect from a Unit Testing book. Such topics include getting PHPUnit installed via PEAR, writing and running some basic tests, doing some configuration of PHPUnit, how to incorporate PHPUnit into your project, how to generate PHPUnit tests semi-automatically using an existing codebase, fixtures and providers and mock objects, how to setup test dependencies, testing abstract classes and traits, how to expect good exceptions and errors, using output buffering for the purpose of testing CLI scripts, how one tests private and protected members, testing database connections, and making sense of the automatically generated code coverage HTML.
The book flows from subject to subject smoothly, with each new topic building on the knowledge of previous topics, without ever introducing the reader to an overly-complex topic. The reader is guided along with concise code examples and explanations of why everything is happening. The author also covers alternative libraries for making Unit Testing easier whenever possible, most notably when covering different mocking frameworks such as Mockery or the authors very-own Phake. Another appreciated set of alternatives include using SQLite for keeping track of data and how the same thing can be done with XML or YAML.
One thing I did wish the book covered more is the why behind Unit Testing, and how to get the reader into the Test Driven Development (TDD) mindset, however this is undoubtedly outside of the scope of a book in the Packt Instant series. Personally, I find myself lacking in the Unit Testing department. The last two large PHP projects I've worked on both had PHPUnit integrated into the codebase and I always find myself writing my Unit Tests at the last minute, well after the code had been finalized.
Several sections of the book did open my eyes to the true potential of PHPUnit, most notably the ability to automatically create unit test skeletons based on existing codebases. This will make for a great way to introduce Unit Testing to existing projects which otherwise lack testing. I was also surprised to see that PHPUnit had a solution for testing Traits, a new feature of PHP 5.4. I did find it interesting that there are standardized methods for testing database connectivity; I had always been under the impression that doing so was taboo.
If you are new to the concept of Unit Testing, and would like to apply it to PHP in particular, I highly recommend picking up a copy of Hands-on Testing with PHPUnit How-to by Michael Lively. It's a small book, quickly digested, and will make a nice desk-side companion (heck, I've already got friends who want to borrow my copy). While the code is thoroughly explained, the content is terse enough to serve as a reference.