$result  = $db->prepare('UPDATE members SET username=:username, password=:password, email=:email, type=:type, purchased=:purchased, expiry=:expiry');

for ($i = 0; $i < count($username); $i++) {
    $result->execute(array(':username' => $username[$i], ':password' => $password[$i], ':email' => $email[$i], ':type' => $type[$i], ':purchased' => $purchased[$i], ':expiry' => $expiry[$i]));
}