The best way to improve your PHP is to write more PHP. Reading a book won't really help much. Think of a problem, and write something to solve it.
For example, I'm working on a wordsearch puzzle generator at the moment. It might sound like a trivial challenge to start with, but once you get into it you'll start having more and more ideas about how to extend it and that pushes the envelope of your PHP skills..
Practise practise practise..