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Grazie mille! È bastato cambiare l'attributo name delle checkbox,

codice:
if ($res_query) { $output = "<form name='renewals' action='index.php?page=00999' method='post'>"; $output.= "Data pagamento: giorno<INPUT Type='text' size='2' Name= 'gg'> mese<INPUT Type='text' size='2' Name= 'mm'> anno<INPUT Type='text' size='4' Name= 'aaaa'>
 Per l'anno: <input Type='text' Name= 'anno' value= '2011'/> 
 Importo: <input Type='float' Name= 'amount' value= '35'> 
"; $output.="<table border='1'>"; $x=0; while ($riga=mysql_fetch_assoc($res_query))   {   $output.="<tr>";   $output.="<td>";   $output.="<input type='checkbox' name='cod[]' value='".$riga['cod']."'  />";   $output.="</td>";   $output.="<td>";   $output.=$riga['nome'];   $output.="</td>";   $output.="<td>";   $output.=$riga['cognome'];   $output.="</td>";   $output.="<td>";   $output.=$riga['data_n'];   $output.="</td>";   $x++;   } $output.="</table>"; $output.="<input type='submit' value='Rinnova iscrizioni'/></form>";  }  echo $output;
Vi devo un caffè