This book, which I wrote with co-authors (and friends) Davey Shafik and Matt Turland, is a mid-level text for existing PHP Developers to raise their game still further. It's not a beginner text, but covers a nice selection of topics which are building blocks to being able to build advanced, robust applications.
There's a chapter on OOP, beginning at the very beginning and then taking a tour through the finer points of the language. Most importantly, it shows how and when to use the various features, and has working code examples to accompany it. To follow on, there's also a chapter on design patterns, also with real examples, and a later section on SPL which is a great way to build elegant object-oriented code in PHP.
One thing I often find myself working with developers on is their databases, data structure and also how to actually talk to the database efficiently. The book has a chapter that covers both aspects, how to normalise data and then how to work with it from PHP using PDO.
All in all, getting together with some peers to work out what we'd tell someone who isn't a beginner, but wants to learn and can handle some quite heavyweight technical ideas, was a lot of fun and personally I think the topics list covers it all quite nicely. I'm recommending this book to everyone looking for an up-to-date text bringing their development skills up a level in a good, all-round way.