MySQL is the most used DataBase in PHP. Just because it's very simple to use, and make information manager very simple.

before starting:

1. What is a DataBase?
DataBase, or DB, is a "information storaging software". It may have client's information, messages, user information, and much more.

2. What is MySQL?

MySQL is one "type" of DataBase. There are a lot of them, Oracle, MSQL, Acess... This article will only teach about MySQL.

Reference Guide
PHP and MySQL functions guide: http://www.php.net/mysql
MySQL official site: http://www.mysql.com