This is a book every PHP developer should have under the bed. I used to read this book everyday and even now I don't code everyday in PHP I still pick it because of how the author explains complex topics in such an easy way.
When PHP switched from spaghetti code to OOP, it took me two books to totally grok what was then a paradigm shift for me. This was the second book I read on the topic and it took me over the finish line.