Although I got the web application book for PHP, I'm starting with the online PHP Manual. It's probably the best step-by-step guide to a new language I've ever seen.

This is also confirming my long-held belief that languages are easy. APIs are hard. PHP is basically a tame perl. Nooooo problem. I bet it's going to take me a while though to get into the flow with the PHP APIs for things like database manipulation, networking, etc. That's where the book I picked up will help.