Probleme cu Paypal IPN Verified/Invalid

Am o problema cu verificarea prin paypal ipn. Dupa efectuarea platiii primesc payment status = Completed si toate celelalte date sunt corecte.
Problema apare cand fac verificarea la paypal, in loc sa primesc raspuns=Verified, primesc raspuns=Invalid, ceea ce nu corespunde realitatii, deoarece tranzactia este corecta. Scriptul e cel de mai jos, l-am actualizat cu ce am gasit pe alte forumuri straine, dar tot nu merge. Ma poate ajuta cineva?

<?php
//read the post from PayPal system and add 'cmd'
$req = 'cmd=_notify-validate';
foreach ($_POST as $key => $value) {
$value = urlencode(stripslashes($value));
$req .= "&$key=$value";
}
//post back to PayPal system to validate
$header = "POST /cgi-bin/webscr HTTP/1.1\r\n";
$header .= "Content-Type: application/x-www-form-urlencoded\r\n";
$header .= "Host: www.paypal.com\r\n";
$header .= "Connection: close\r\n";
$header .= "Content-Length: " . strlen($req) . "\r\n\r\n";
$fp = fsockopen ('ssl://www.sandbox.paypal.com', 443, $errno, $errstr, 30);
//
//error connecting to paypal
if (!$fp) {
//
}
  
//successful connection  
if ($fp) {
fputs ($fp, $header . $req);
  
while (!feof($fp)) {
$res = fgets ($fp, 1024);
$res = trim($res); //NEW & IMPORTANT
  
if (strcmp($res, "VERIFIED") == 0) {
//insert order into database
}
  
if (strcmp ($res, "INVALID") == 0) {
//insert into DB in a table for bad payments for you to process later
}
}
fclose($fp);
}
?>