Check out
Amazon.com: PHP and MySQL for Dynamic Web Sites: Visual QuickPro Guide (2nd Edition) (Visual QuickPro Guide): Books: Larry Ullman
When I started doing web design, I didn't know much about MySQL, and nothing about PHP, but this guide helped me with both. It has a lot of examples, including creating a username/password system powered by MySQL, and even covers some of the security issues involved.
Not a plug, just a resource that worked for me. I have seen a lot of tutorials, but none that were as good a reference as the book.