[Eikinskjaldi]

Well, I dont know why you are getting the error, so please post a full version of the error you are getting here. here's some better code with an explanation.

PHP Code:

//this first line grabs userid from the form/page that the user submitted
//and "cleans" it...makes sure it is an integer.  This is a security thing.
$vbulletin->input->clean_gpc("p","userbarid",TYPE_INT);

//Now lets grab the data from the database
$userbarquery=$db->query_read("SELECT userbarid, userbarpath FROM ".TABLE_PREFIX."userbar WHERE userbarid='".$vbulletin->GPC['userbarid']."'");

//if there was no data, exit with an approproriate error page
    if(!$db->num_rows($userbarquery)){
        eval(standard_error(fetch_error('unvalid_userbar')));
    }

//if there was data, retrieve it. (assume only one row of data)
$userbardata = $db->fetch_array($userbarquery);

//$userbardata is an array, with members  $userbardata['userbarid'] and $userbardata['userbarpath'].  The former is identical to $vbulletin->GPC['userbarid']

//now write out the data
    $db->query_write("UPDATE ".TABLE_PREFIX."user SET userbar='".$vbulletin->GPC['userbarid']."', userbarpath='".$userbardata['userbarpath']."' WHERE userid='".$vbulletin->userinfo['userid']."'");
    $vbulletin->url = 'usercp.php' . $vbulletin->session->vars['sessionurl_q'];
    eval(print_standard_redirect('redirect_userbarselected'));