Although I did not read it, yet, I'd like to recommend it to you, since I think to know the authors good enough to believe they write good books.

In short, "Exploring PHP" is a roundup about advanced PHP programming experiences, covering different topics like: Event driven programming in PHP, Generation of 3D graphics, Refactoring and Migration, PHP6.

From the little that he's looked into the book, he can already tell it's a good resource and is recommending it based on that and the authors it comes from.

I have been working on this together with Frank, Christian, Sandro and Kore and the book features five articles on advanced PHP topics. All articles in the book have been written in German.

He specifically mentions a part he contributed - a look at event-driven development (for component applications). The book also talks about using various PEAR packages and SimpleTest. The planned release date is August 17th from Entwickler Press (published in German).