I taught myself PHP using Larry Ullman's "PHP for the World Wide Web" and then went on to his "PHP and MySQL for the World Wide Web". I found them both easy to read and very clear. If you already know a little you could probably skip to the second book. I also like PHP Solutions by David Powers. As a quick reference, I use Teach Yourself PHP. Online there is the excellent W3Schools PHP reference, which is much better than the official PHP reference, which seems to have been written for robots.
Hope this helps.