I learned from the PHP Manual
and usually use it as a reference.
You might also try W3Schools http://www.w3schools.com/php/default.asp
. I haven't actually explored their php content much, but their site in general is excellent.
I'm assuming that you are looking to work on your own sites and will learn based on what you want to do. If you are hoping to become a professional web programmer for a company that builds web applications, as soon as you get a basic understanding of php, I recommend focusing on object-oriented php, as that is what most of the people hiring right now are looking for.