Where can I learn about PHP & MySQL?

First there are several resources on the web that have tons of free tutorials and forums where the most commons questions have already been answered. You should check out www.phpbuilder.com, which will have lots of information about MySQL as well. You should also check out www.php.net and www.zend.com. There a plenty more websites and articles on the web but you can find links to all of those if you check out these sites.

If you're looking for books, a good introduction is PHP and MySQL Web Development by Welling and Thomson. It covers a very broad range of topics and is helpful if you have not done a lot of web programming, but you should at least be familiar with HTML and the most basic concepts of programming. If you want books that assume you already have a web programming background or books that cover more advanced topics I would suggest:
- PHP Developer's Dictionary by Wyke, Walker and Cox (a great language reference when you just need to know how a particular function works)
- Web Application Development with PHP 4.0 by Ratschiller and Gerken (helpful for advanced developers who need to develop robust enterprise applications)
- MySQL by DuBois (helpful if you really need to know the ins and outs of MySQL development, administration and performance tweaking)
- PHP Developer's Cookbook by Hughes and Zmievski (a set of proven recipes for accomplishing common tasks in PHP)
- A Programmer's Introduction to PHP 4.0 by Gilmore (a task oriented introduction/reference for PHP)