Paypal IPN on Sandbox mode

Paypal IPN on Sandbox mode

I had installed IPN on my account which seems to be working on IPN
simulator, but when testing on Sandbox mode, it is not working. This is my
code, am I having something wrong?
<?php
$req = 'cmd=_notify-validate';
foreach ($_POST as $key => $value) {
$value = urlencode(stripslashes($value));
$req .= "&$key=$value";
}
//Set up the acknowledgement request headers
$header .= "POST /cgi-bin/webscr HTTP/1.0\r\n";
$header .= "Content-Type: application/x-www-form-urlencoded\r\n";
$header .= "Host: www.sandbox.paypal.com\r\n";
$header .= "Content-Length: " . strlen($req) . "\r\n\r\n";
//Open a socket for the acknowledgement request
$fp = fsockopen ('ssl://www.sandbox.paypal.com', 443, $errno, $errstr, 30);
// Post request back to PayPal for validation
fputs ($fp, $header . $req);
while (!feof($fp)) { // While not EOF
$res = fgets ($fp, 1024); // Get the acknowledgement
response
if (strcmp ($res, "VERIFIED") == 0) { // Response is VERIFIED
// Send an email announcing the IPN message is VERIFIED
$mail_From = "IPN@example.com";
$mail_To = "me@gmail.com";
$mail_Subject = "VERIFIED IPN";
$mail_Body = $req;
mail($mail_To, $mail_Subject, $mail_Body, $mail_From);
}
else if (strcmp ($res, "INVALID") == 0) { // Response is INVALID
// Notification protocol is NOT complete, begin error handling
// Send an email announcing the IPN message is INVALID
$mail_From = "IPN@example.com";
$mail_To = "me@gmail.com";
$mail_Subject = "INVALID IPN";
$mail_Body = $req;
mail($mail_To, $mail_Subject, $mail_Body, $mail_From);
}
}
fclose ($fp);
?>
Is it something that I have wrong in the code or is it a problem from
Paypal? How can I solve this problem?
On other hand, my account is verified with Paypal