Renumbering items in a list with SQL queries?
$consulta = "UPDATE `list`
                SET `pos` = $pos
              WHERE `id_item` IN (SELECT id_item
                                    FROM lists
                                   WHERE pos = '$item'
                                ORDER BY pos DESC
                                   LIMIT 1)
                AND id_usuario = '$us'
                AND id_list = '$id_pl'";

$consulta = "UPDATE `list`
                SET `pos` = $pos
              WHERE `$pos` = '$item'
                AND id_usuario = '$us'
                AND id_list = '$id_pl'";

AND id_list = '$id_pl'

foreach ...
    ...$pos, $item, $us...

    UPDATE list
       SET pos = $pos + 100
     WHERE id_item = '$item'
       AND id_usuario = '$us'
       AND id_list = '$id_pl'
       AND pos < 100
end foreach

UPDATE list
   SET pos = pos - 100
 WHERE id__list = '$id_pl';