These functions allow you to access MySQL database servers. In order to have these functions available, you must compile php with MySQL support by using the --with-mysql option. If you use this option without specifying the path to MySQL, php will use the built-in MySQL client libraries. Users who run other applications that use MySQL (for example, running php3 and php4 as concurrent apache modules, or auth-mysql) should always specify the path to MySQL: --with-mysql=/path/to/mysql. This will force php to use the client libraries installed by MySQL, avoiding any conflicts.
More information about MySQL can be found at http://www.mysql.com/.
Documentation for MySQL can be found at http://www.mysql.com/documentation/.