How can I prevent paypal duplicate payment using txn_id? - php

Question

So I am using PHP cURL and PayPal IPN.
I don't want my customer to pay twice by mistake. I have heard that I can do that using txn_id but I don't know how.
Here is my code ...
<?php
// connecting to database
// Prepare the URL to send via cURL
$req = 'cmd=_notify-validate';
if(function_exists('get_magic_quotes_gpc')) {
$get_magic_quotes_exists = true;
}
foreach ($_POST as $key => $value) {
if($get_magic_quotes_exists == true && get_magic_quotes_gpc() == 1) {
$value = urlencode(stripslashes($value));
} else {
$value = urlencode($value);
}
$req .= "&$key=$value";
}
// Initial cURL
$ch = curl_init();
// Set opt
curl_setopt($ch, CURLOPT_URL,"https://www.paypal.com/cgi-bin/webscr");
curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $req);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
curl_setopt($ch, CURLOPT_FORBID_REUSE, 1);
// Return result
$result = curl_exec($ch);
// Close cURL connection
curl_close($ch);
// If condition
if($result == "VERIFIED"){
// perform database update
}
?>
How can I prevent the buyer from paying more than once?

Answer 1

The code you provided here is for PayPal IPN, which I think could help little to avoid duplicated payment. Instead, I recommend you adding invoice into your payment data. As PayPal system by default blocks payments with same Invoice ID, I hope this could help you avoid your customers from paying twice by mistake.
If you are integrating PayPal Payment Standard, please check below page for variable list.
https://developer.paypal.com/webapps/developer/docs/classic/paypal-payments-standard/integration-guide/Appx_websitestandard_htmlvariables/
If you are integrating PayPal Express Checkout, please check below pages.
https://developer.paypal.com/docs/classic/api/merchant/SetExpressCheckout_API_Operation_NVP/
https://developer.paypal.com/docs/classic/api/merchant/DoExpressCheckoutPayment_API_Operation_NVP/