Hasin Hayder has asked the PHP community what some of their favorite books have been for learning PHP and related technologies:

So far I’ve read a lot of books written on PHP. But when someone asks me which one is my favorite, I get three or may be four which were very good. [...] Of course there are other books which are very good in fact. But these are my favorite (Well, you see that Beginning PHP4 and Professional PHP4 are obsolete by now, but they helped me a lot that time) [...] What is your most favorite Book on PHP? It doesn’t matter if they are obsolete or 10 years old – Just share you most favorite one with us.

The comments included a wide range of titles from O'Reilly's "PHP Cookbook" and "PHP in Action" out to the classic "PHP Objects, Patterns, and Practice". There's many more besides these so if you're looking for a good book and a place to start, check out the comments for some great suggestions.