<form>
while ($row = mysql_fetch_*()) {
  <input name="firstname[<?=$row["id"]?>]" value="<?=$row["firstname"]?>" />
  <input name="lastname[<?=$row["id"]?>]" value="<?=$row["lastname"]?>" />
}
<input type="submit" />
</form>

$_POST = array(
  "firstname" => array(5 => "Max", 6 => "Moritz"),
  "lastname" => array(5 => "Müller", 6 => "Meyer"));