On the Listmania site there's a recent post introducing you to ten PHP books that can help you to master the language and move forward in your development skills.

PHP is the widely-used, free, and efficient alternative to competitors such as Microsoft’s ASP. If you are just starting up trying to learn PHP then this post is just what you’re looking for as it list 10 really good books that will get you started well.

Books included in their list of ten are things like:

• PHP and MySQL Web Development (4th Edition)
• PHP Cookbook : Solution and Examples for PHP Programers
• Head First PHP & MySQL
• Beginning PHP and MySQL
• PHP Objects, Patterns, Third Edition

Obviously these books are for developers at different levels in their skillsets, but you can pick out the ones from this list that might suit you best. My personal recommendation is for "PHP Objects, Patterns and Practice" by Matt Zandstra.