int mysql_change_user (string user, string password [, string database [, int link_identifier]])
mysql_change_user() changes the logged in user of the current active connection, or the connection given by the optional parameter link_identifier. If a database is specified, this will default or current database after the user has been changed. If the new user and password authorization fails, the current connected user stays active.
Note: This function was introduced in PHP 3.0.13 and requires MySQL 3.23.3 or higher.