Classes Question

I'm hoping somebody has the answer to this! I have 3 pages, one a normal PHP page, and 2 which are classes. One class, is a MySQL class I have written to connect/update users, etc And the other is the manage users on the system. Is there a way I can get the "users" class to talk to the "mysql" class? I have this code to make a new instance of the MySQL class (the True means use persistent connections) and a new instance of the users class $db = new MySQL(True); $users = new Users(); I am then using this code to check the list of banned users $users->CheckBannedUser("spacetowns",$db); In the CheckBannedUser function, I have this $db->sql_query("SELECT user FROM banned_users"); But that does not work, (it works fine when used from the standard PHP page) Is it not possible to do what I am trying to do? Or am I doing it all wrong? Many thanks, Paul