I found "Professional PHP Programming" (WROX) to be quite helpful.
And I agree with frigidman, www.php.net can be very useful, but it's sometimes a crapshoot. What I do like about php.net is that it's current, and two of my three PHP programming books are not. If I need a function prototype for something brand, spankin' new, it's there where it's not in any of my books.
I have three books on PHP programming, there's things that like and dislike about each one. I have 'Core PHP Programming' by Leon Atkinson, and I use it a lot because the index is excellent. But I've found some code examples in that book that were just plain wrong. I also have "Building Web Applications with PHP3" by David Medinets, some excellent concepts are covered, but I find the author's coding style difficult to read.
IMHO (and O's are like A's, we've all got 'em), MySQL by Paul DuBois (New Riders) is the MySQL reference. A very, very well written reference for MySQL, with some excellent PHP examples.