PHP is pretty easy. If you know C, you should be able to pick it up just by looking at source code. There's no shortage of PHP help sites, which Google can help you find. The PHP manual itself is pretty nicely written, especially the online version which allows comments and suggestions from other readers.
To be honist, if you're a pro with C, then PHP will be a breeze. A great book is PHP, MySQL and Apache by Julie C. Meloni. I was confident with PHP in a few days. The www.w3schools.com PHP tutorial is good apparently too.