function slf_GetUserGroupInfo()
{
    global $db;
    $Result = array();
    $Fetch = $db->query_read
    (
        'SELECT '.
            'usergroupid, '.
            'usertitle, '.
            'opentag, '.
            'closetag '.
        'FROM usergroup'
    );
    while($Row = $db->fetch_array($Fetch))
    {
        $Result[$Row['usergroupid']]['usertitle'] = $Row['usertitle'];
        $Result[$Row['usergroupid']]['opentag'] = $Row['opentag'];
        $Result[$Row['usergroupid']]['closetag'] = $Row['closetag'];
    }
    return($Result);
}

function slf_FormatForumName($ForumId, $UserGroupInfo)
{
    global $db;
    $Fetch = $db->query_read
    (
        'SELECT '.
            'username, '.
            'IF(displaygroupid=0, usergroupid, displaygroupid) AS usergroup '.
        'FROM user '.
        'WHERE userid = '.($ForumId * 1).' '.
        'LIMIT 1'
    );
    $Row = $db->fetch_array($Fetch);

    // Format name.
    $Group = $Row['usergroup'];
    $OpenTag = $UserGroupInfo[$Group]['opentag'];
    $CloseTag = $UserGroupInfo[$Group]['closetag'];
    return($OpenTag.$Row['username'].$CloseTag);
}

////////////////////////// USAGE ////////////////////////////

function mysub()
{
    // First call this at top of the sub
    $UserGroupInfo = slf_GetUserGroupInfo();

    // This is the vBulletin User ID we want to get the name for.
    $TheUserId = 6;

    // Not only get the name, but format it so it looks like a real vBulletin username.
    $ClickableFormattedName = '<a href="/member.php?u='.$TheUserId.'">'.slf_FormatForumName($TheUserId, $UserGroupInfo).'</a>';

    // Do something with it.
    echo $ClickableFormattedName;
}