PayPal IPN PHP Handshake Error

Question

Ok, so I have recently started trying to work with PayPals IPN, I have read some of PayPals pages on their IPN and used the PHP source from here: https://developer.paypal.com/docs/classic/ipn/gs_IPN/, I have ended up with this entire code:

<?php

header('HTTP/1.1 200 OK');

$req = 'cmd=_notify-validate';

foreach ($_POST as $key => $value) {
    $value = urlencode(stripslashes($value));
    $req .= "&$key=$value";
}

$header = "POST /cgi-bin/webscr HTTP/1.1\r\n";

$header .= "Content-Type: application/x-www-form-urlencoded\r\n";
$header .= "Content-Length: " . strlen($req) . "\r\n\r\n";

$header .= "Host: www.sandbox.paypal.com:443\r\n";

$fp = fsockopen ('ssl://www.sandbox.paypal.com', 443, $errno, $errstr, 30);

fputs($fp, $header . $req);

while (!feof($fp))
{
    $res = fgets($fp, 1024);
    if (strcmp ($res, "VERIFIED") == 0) {  

    } else if (strcmp ($res, "INVALID") == 0) { 

    }

    fclose ($fp);
}
?>

When I use PayPal's IPN simulator it says

IPN was not sent, and the handshake was not verified. Please review your information.

This is strange because it seems as though i read PayPal's PHP documentation correctly and this code seems like it should work? What could Possibly going wrong?

Answer 1

Try logging in back-end. Sometimes PayPals IPN listener says handshake was invalid even though it was.