I think if you are going to use query instead of query_first you need to use 'while' like this:
PHP Code:
$newestmembers = $DB_site->query("
SELECT userid, username
FROM user
ORDER BY userid DESC
LIMIT 5
");
while ($shownewest=$DB_site->fetch_array($newestmembers))
{
$counter++;
$newuserid = $shownewest['userid'];
$newusername = $shownewest['username'];
$newestbit .= "<a href=\"member.php?u=$newuserid\">$newusername</a>";
if ($counter < 5)
{
$newestbit .= ", ";
}
}