I'd dabbled with Perl a while back and done quite a bit, and spent the last year developing with ColdFusion.. so my background was I knew what I wanted to do, I just needed to know how to do it with PHP..
I bought Web Application Development with PHP 4.0 (New Riders) - it's not bad but maybe a bit advanced.. go with it once you've done a whole load with PHP already and want to take it further.
Fast and Easy PHP Development (or something?) by Julie Meloni.. great for beginning PHP.. I found it a bit simplistic given my background, but I gave it to my dad and he loves it.. bit basic and repetitive in places, but a great hand holder.. added bonus is that it comes with a CD with MySQL and PHP on it and has instructions for installing under Linux or Win95/98/NT.
My award for the best book I've bought on PHP so far goes to Professional PHP Programming (Wrox). Just about the right level for me, tells me what I need to know.. only gripe was there's not a lot about sessions in there which was something I wanted to learn about but I've found enough stuff on the web to keep me happy. I think there's also a Wrox book called Beginning PHP which is probably also worth a look.
Just my £0.02! :-)