What does MYSQL mean in the following code.
require_once (MYSQL);
Looks to me like MYSQL is a constant. Look for something like this:
define('MYSQL', '/some/path/');
to get the 'real' include.
It's most likely a constant defined via define().
MYSQL would be a constant, defined by somewhere saying:
define("MYSQL", "/path/to/the/mysql/file.php");
Then, when they go to include/require it, it takes the defined path, and include/requires it.
It's (almost assuredly) referring to a constant that's define()d elsewhere.