this site, obviously, is THE one-stop shop. extremely valuable.
the php.net manual pages have comments below most of the functions; they're harder to follow in some cases than a board like this, but still useful. and it's just good to get familiar with it.
Hughes: PHP Developer's Cookbook, pub. by SAMS, is good after you get a little more comfortable.
The Peachpit Press book by Larry Ullman is good for the basics.
There are also several books on MySQL that help to put PHP in a database context; that also helps things to click into place. In particular, pay close attention to anything by New Riders publishing.
anyway, those are my suggestions. read this board every day and you'll pick up volumes of great information.
good luck. knelson.