[ArbuZz]

Thank you. And is there a place where all db functions of vBulletin are listed?

For example what's wrong with this code:

...
require_once('./includes/init.php'); // includes class_core.php
require_once('./includes/class_dm.php'); // for class_dm_user.php
require_once('./includes/class_dm_user.php'); // for user functions
require_once('./includes/functions.php'); // vbsetcookie etc.
require_once('./includes/functions_login.php'); // process login/logout
...
global $vbulletin;
$useridq = $vbulletin->db->query_read("SELECT userid FROM "
. TABLE_PREFIX . "user WHERE username='{$username}'");
if (!$useridq) return $useridq;
$userid = $useridq['userid'];

DB query just returns nothing originally $vbulletin->db->query_read was $vbulletin->db->query_first_slave. But it doesn't work either. How can one print out the error if it happened upon query?