MySQL is a relational database with a license that allows you to use it cost-free for most noncommercial purposes. It shares many features with mSQL because it was originally conceived as a faster, more flexible replacement. Indeed, MySQL has delivered on these goals. It easily outperforms even commercial databases. Not surprisingly, MySQL is the database of choice for many PHP developers.
To find out more about MySQL as well as obtain source code and binaries, visit the Web site at <http://www.mysql.com/>. There are plenty of mirrors to aid your download speed. If you're looking for a printed text on MySQL, please consider Core MySQL.
The MySQL extension was written by Zeev Suraski.