MySQL (
www.mysql.com) is the world's most popular, and some might argue, best open source database. In fact, particularly as version 4 adds new features, MySQL is a viable competitor to the pricey goliaths such as Oracle and Microsoft's SQL Server. Like PHP, MySQL offers excellent performance, probability, and reliability, with a moderate learning curve and little to no cost.
Text Originally From "A Visual QuickPro Guide to PHP and MySQL" by Larry Ullman
^ Very recommended reading if you are looking to learn PHP.