Question

I am trying to create a web survey questionaire, but I don’t want same people using same ip to submit it more than once within an hour time, I am using php, I assume I need to use
$_SERVER['REMOTE_ADDR'] to get the client ip and store in the session or database and then use it to compare the new ip, I am not sure if it is right and don’t know how to exact implement this in php, can anyone help me with it, thanks in advance!

Answer 1

When survey is submitted:

 /*
   Tracking table structure:
    `id`              INT(11) unsigned NOT NULL AUTO_INCREMENT
    `client_ip`       VARCHAR(15) NOT NULL
    `submitted_time`  DATETIME NOT NULL
 */

 $query = "SELECT count(`id`) AS 'count'
           FROM `tracking_table`
           WHERE 
             `client_ip` = '".mysqli_real_escape_string($link, $_SERVER['REMOTE_ADDR'])."'
             AND `submitted_time` > '".date('Y-m-d H:i:s',strtotime('-1 hour'))."'
           LIMIT 1";
 $result = mysqli_fetch_assoc(mysqli_query($link, $query));

 if ($result['count'] > 0) {
   echo "You have already submitted within the last hour";
   exit;
 }

 // process survey here

 $query = "INSERT INTO `tracking_table`
             (`client_ip`, `submitted_time`)
           VALUES
             ('".mysqli_real_escape_string($link, $_SERVER['REMOTE_ADDR'])."', ".date('Y-m-d H:i:s').")";
 mysqli_query($link, $query);

However, you may find that this is not a good requirement – there are many valid situations where multiple users may use the same IP address (such as, for example, student accomodation). You may be preventing valid submissions by imposing this limit.

EDIT

Here is a basic outline of how you might do this with cookies (taking into account the limitations discussed below).

Our cookie system works on a pair of values. ckv_1 will hold the timestamp at which the last survey was submitted. ckv_2 will hold a hash based on the timestamp and a salt, in an effort to stop people from screwing with the cookies. Obviously, if both cookies are deleted, we won’t be able to detect it, but at least this provides some sort of validation:

function get_cookie_hash ($timestamp, $salt) {
  return md5("Extra random static string; TS: $timestamp; Salt: $salt; Extra random static string.");
}

$cookieSalt = 'Th1si54rAnd0MsTr1nG!';

// If at least one of the cookies was returned, validate the request
if (!empty($_COOKIE['ckv_1']) || !empty($_COOKIE['ckv_2'])) {

  $valid = FALSE;

  do { // Wrapped in a do-while to allow us to break out easily
    // Make sure both value are set
    if (empty($_COOKIE['ckv_1']) || empty($_COOKIE['ckv_2'])) break;
    // Get old timestamp as integer
    $oldTS = hexdec($_COOKIE['ckv_1']);
    // Make sure timestamp is more than one hour old, and the hash cookie matches it
    if ($oldTS > (time() - 3600) || $_COOKIE['ckv_2'] != get_cookie_hash($oldTS, $cookieSalt)) break;
    // OK if you get here
    $valid = TRUE;
  } while (FALSE);

  if (!$valid) {
    echo "Sorry - you cannot submit a survey more than once in an hour.";
    exit;
  }

}

// process survey here

// Set the tracking cookies after processing (but before any output!)
// We'll set them as HTTP only to help prevent XSS-type attacks
$cookieTime = time();
setcookie('ckv_1', dechex($cookieTime), 7200, '', '', FALSE, TRUE);
setcookie('ckv_2', get_cookie_hash($cookieTime, $cookieSalt), 7200, '', '', FALSE, TRUE);