Ask HN: The best PHP book for Ruby programmer?

I need to learn some PHP after several years of serious hacking in Ruby. Of all Ruby books I've read "The Ruby Way" by Hal Fulton was the most practical and fun to follow. For example, while telling you about basic data structures it would also give you a recipe for implementing a sparse matrix or how to use arrays as mathematical sets.

What is the best book that focuses on PHP as a programming language, rather than on building websites?