A book called "Head First PHP & MySQL" is the first book I read that finally allowed me to start making progress with PHP. While the book is a good reference book, it is really best to take the time to go through the book and do all the exercises and take it all in.
O'Reilly books are excellent resource in learning PHP, you can find some here.. http://oreilly.com/php/index.html#Bestsellers and yes you can find absentx's suggested book there..
Are you looking for just general knowledge or anything specific? There's several online resources that are far better than the available books.
When i began to learn, I used to use the Visual Quickstart series, but another good series of books is the SAMS teach yourself in 24 hours.
I would say general knowledge for now, beginning to intermediate. Most of the books is ok, but some of them are better than others. Somebody already recommended Objects, patterns and practice somewhere else. It's got to be a good one... Thanks.