Integrate with WP

Hi All,

I am trying to put the punbb 1.2.10 login screen (only the interface, not sharing the user db) in the the page of wordpress 1.5.2. So that I

    define('PUN_ROOT', './forum/');
    include './forum/include/common.php';

Actually, I follow these steps http://punbb.org/forums/viewtopic.php?id=8786 to integrate the login. If I create a new blank php file to contain the about codes, it run very well. But if I put the codes inside of the wp's theme, I got the following problem.

I traced that the function check_cookie & set_default_user cannot find the global $db, because I tried to add
   
    echo echo $db->prefix;

inside of these function but it did not echo anything.

Please advice.

Thanks

gchoyy