Codice PHP:
                                <tr><?php $k=0
                                             do { 
?>
                                                <td height="22">
                                                <?php $k++; ?>
                                                <?php $p="periodi".$k ?>
                                                <input name="<?php echo $p?>" type="hidden" value="<?php echo $row_periodi['tempo']; ?>" /><?php echo $row_periodi['tempo']; ?></td>
                                                <?php } while ($row_periodi mysql_fetch_assoc($periodi)); ?>
controlla questo do... al primo passaggio non ha niente da stampare nel value..
riformulalo con un while con controllo in testa..magari cambia qualcosa