check if mysql row exists

[Nullifi3d]

I need to check if a mysql row exists for the visiting user ($vbulletin->userinfo['userid']). I am using the below code but am getting an error at the top of the pages.

PHP Code:

if (!isset($_REQUEST['do'])) {
    $uid = $vbulletin->userinfo['userid'];
    $check = $db->query_first("SELECT * FROM " . TABLE_PREFIX . "banners WHERE userid = '$uid'");
    $result = mysql_num_rows($check);
    if ($result !== "0") {
        $bannerinfo = $db->query_first("SELECT status FROM " . TABLE_PREFIX . "banners WHERE userid = '$uid'");
        if ($bannerinfo[status] == "declined") {
            $_REQUEST['do'] = "add";
        } else {
            $_REQUEST['do'] = "statistics";
        }
    } else {
        $_REQUEST['do'] = "add";
    }
} 


Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /banners.php on line 58