允许通过IPN访问特定站点

How is access granted to a specific site by IPN? I am using a site called paygol and people can make donation or sales by SMS. When you create the button for a donation they give you an IPN code you can modify.

    <?php 

    // check that the request comes from PayGol server
    /*
    if(!in_array($_SERVER['REMOTE_ADDR'],
      array('109.70.3.48', '109.70.3.146', '109.70.3.210'))) {
      header("HTTP/1.0 403 Forbidden");
      die("Error: Unknown IP");
    }
    */ 
// CONFIG 
$your_service_id = 49001;  // this is my service ID from Paygol     

    // get the variables from PayGol system
    $message_id = $_GET['message_id'];
    $service_id = $_GET['service_id'];
    $shortcode  = $_GET['shortcode'];
    $keyword    = $_GET['keyword'];
    $message    = $_GET['message'];
    $sender = $_GET['sender'];
    $operator   = $_GET['operator'];
    $country    = $_GET['country'];
    $custom = $_GET['custom'];
    $points = $_GET['points'];
    $price  = $_GET['price'];
    $currency   = $_GET['currency'];

    // Here you can do whatever you want with the variables, for instance inserting or updating data into your Database 

    ?>

How can I allow access to one a specific page once a donation is made?

set encrypted or hashed data into cookie (1 or 2 years of expiration) that tells that he had made donation:

$token = md5($_SERVER['REMOTE_ADDR'].time());
mysql_query("INSERT INTO access_tokens (token) VALUE ('".mysql_real_escape_string($token)."')");
setcookie("access_token", $token, time() + 365*24*60*60);

and in the page check if it's token exist in access_tokens table:

$token = $_COOKIE['access_token'];
$accept = false;
if(strlen($token) == 32) {
  $r = mysql_query("SELECT 1 FROM access_token WHERE token = '".mysql_real_escape_string($token)."' LIMIT 1");
  if(mysql_num_rows($r) == 1) {
    $accept = true;
  }
}

if(!$accept) {
  header('Location: /');
  exit(0);
}

there are many methods to make so.

one of them to send specific COUPON CODE to user's email after making donation and ask everytime before giving access to the resource.

it all depends on Your imagination (: