Because of the more important role of OO in PHP5 and the fact that it's implementation is far better than in previous PHP version, I would recommend reading up on some generic programming knowledge in the form of Design Patterns.
There is a PHP specific book on Design Patterns, available at www.phparch.com
Applying Desing Patterns to your code, will drastically improve the maintainability, readability and quality of your code. And it's quite fun, once you get the hang of it.