Normally I wouldn't recommend books like this, but I recently bought "PHP & mySQL for Dummies" to learn PHP. I picked up a second hand copy on eBay for under a tenner.
Within 3 months I've gone from nothing to having built a (nearly) complete PHP
website hosting with a shopping cart, photo upload facility, members-only area etc. It was really well written and very accessible. Once you've got the basics of PHP, their
website hosting (
www.php.net) has loads of information on more advanced functions, and good examples from other developers.
I would recommend buying a book covers combined PHP and mySQL together, because it make a lot of sense when you think about what you are most likely to use PHP for - reading from and inserting into a mySQL database.
Good luck!