ho provato sia così:

(1email, 2password, 3`expires`, 4phone, 5cognome, 6cell)
VALUES (1%s, 2%s, 3DATE_ADD(NOW(), INTERVAL ".$credit['days']." DAY, 4%s, 5%s, 5%s)",

ho messo il numero davanti ad ognuno.

Codice PHP:
$insertSQL = sprintf("INSERT INTO info (email, password, `expires`, phone, cognome, cell) VALUES (%s, %s, DATE_ADD(NOW(), INTERVAL ".$credit['days']." DAY, %s, %s, %s)",         GetSQLValueString($_POST['email'], "text"),                          GetSQLValueString(md5($password), "text"),                         GetSQLValueString($_POST['expires'], "date"),                                                GetSQLValueString($_POST['phone'], "text"),                         GetSQLValueString($_POST['cognome'], "text"),                                      GetSQLValueString($_POST['cell'], "text")); 
che così:

(1email, 2password, 3`expires`=DATE_ADD(NOW(), INTERVAL ".$credit['days']." DAY, 4phone, 5cognome, 6cell)

VALUES (1%s, 2%s, 3%s, 4%s, 5%s, 6%s)",


Codice PHP:
$insertSQL = sprintf("INSERT INTO info (email, password, `expires`=DATE_ADD(NOW(), INTERVAL ".$credit['days']." DAY, phone, cognome, cell) VALUES (%s, %s, %s, %s, %s, %s)",         GetSQLValueString($_POST['email'], "text"),                          GetSQLValueString(md5($password), "text"),                         GetSQLValueString($_POST['expires'], "date"),                                                GetSQLValueString($_POST['phone'], "text"),                         GetSQLValueString($_POST['cognome'], "text"),                                      GetSQLValueString($_POST['cell'], "text")); 
ma da errore, cosa sbaglio?

grazie