fsockopen returns Bad Request (Invalid hostname) in one server, but works fine in others

Hello.

I have a paymento module developed for oscommerce, working well in several servers, but got a problem in a specific setup:

1. I can't use cURL, as it is not avaiolable in this server;
2. using fsockopen to access a host, the script connects, but only in http (non secure) mode;
3. Passing the header as:
 } elseif ($fsocket == true) {
    $Cabecalho  = "POST /Security/NPI/Default.aspx HTTP/1.1\r\n";
    $Cabecalho .= "Content-Type: application/x-www-form-urlencoded\r\n";
    $Cabecalho .= "Content-Length: " . strlen($BRpay) . "\r\n\r\n";

 
    if ($fp || $errno>0){
        fputs ($fp, $Cabecalho . $BRpay);
        $confirma = false;
        $resp = '';
        while (!feof($fp)){
            $res = @fgets ($fp, 1024);
            $resp .= $res;
           
            if (strcmp ($res, "VERIFICADO") == 0)
            {
                $confirma=true;
                debug_var ($resp, "fsockopen: ".date("Y-m-d G:i:s"),true);
                break;
            }
        }
        fclose ($fp);
    } else{
        echo "$errstr ($errno)<br />\n";
             debug_var ($errstr.'('.$errno.')', "erro em fsockopen:: ".date("Y-m-d G:i:s"),true);
        // ERRO HTTP
      }
  }

I have the following answer:

HTTP/1.1 400 Bad Request
Content-Type: text/html
Date: Sat, 02 Dec 2006 18:18:27 GMT
Connection: close
Content-Length: 39

<h1>Bad Request (Invalid Hostname)</h1>

The string $BRpay contains the data to be validated in the server. The same data works in my test server.

This scripts works well in many environments, but in this specific one, it doesn't:
* PHP 4.4.2
* Linux Server
* MySQL 4.1.2
* Server protocol HTTP/1.1

I hope someone can help me, as this problem prevents my client to use this payment mode, and I need to fix it ASAP.

Thanks!