PayPal IPN Bad Request 400 Error

Using the PayPal IPN, I keep getting an error 400.

I have been making the script send me emails of $res to see what the response is, inside of the while (!feof($fp)) {} loop. I always end up getting the error: HTTP/1.0 400 Bad Request

In total I get back:

HTTP/1.0 400 Bad Request
​Connection: close
Server: BigIP
Content-Length: 19
​Invalid Host Header

​The last line after this is just blank. Here is my code, I have tried changing loads of things but nothing works.

$req = 'cmd=_notify-validate';
foreach ($_POST as $key => $value) {
$value = urlencode(stripslashes($value));
$value = preg_replace('/(.*[^%^0^D])(%0A)(.*)/i','${1}%0D%0A${3}', $value);// IPN fix
$req .= "&$key=$value";
}

// post back to PayPal system to validate
$header = "POST /cgi-bin/webscr HTTP/1.0\r\n";
$header .= "Content-Type: application/x-www-form-urlencoded\r\n";
$header .= "Content-Length: " . strlen($req) . "\r\n\r\n";

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

if (!$fp) {
// HTTP ERROR
} else {
   fputs($fp, $header . $req);
   while (!feof($fp)) {
       $res = fgets ($fp, 1024);
       if (strcmp ($res, "VERIFIED") == 0) {
           //ADD TO DB
       } else if (strcmp ($res, "INVALID") == 0) {
           // PAYMENT INVALID & INVESTIGATE MANUALY!
           // E-mail admin or alert user
       }
   }
   fclose ($fp);
}

I have added a line, this is the header before it is sent:

 Host: www.sandbox.paypal.com
 POST /cgi-bin/webscr HTTP/1.0
 Content-Type: application/x-www-form-urlencoded
 Content-Length: 1096