The mysqli extension allows you to access the functionality provided by MySQL 4.1 and above. More information about the MySQL Database server can be found at » http://www.mysql.com/
An overview of software available for using MySQL from PHP can be found at Overview
Documentation for MySQL can be found at » http://dev.mysql.com/doc/.
Parts of this documentation included from MySQL manual with permissions of MySQL AB.
All Examples in the MySQLI documentation use the world database from MySQL AB. The world database can be found at » http://dev.mysql.com/get/Downloads/Manual/world.sql.gz/from/pick