file_get_contents with HTML log-in form only

Hi,

I use this module with authentication type «HTML log-in form» only. Not the HTTP basic or digest.
I develop some pages that use the PHP function file_get_contents() and also the Drupal function drupal_http_request. But in the both cases, Drupal is not allowed to get the content of the page. I only get the HTML login form source. I tried this without success:

$url = "http://example.com/page/my-page";

$postdata = http_build_query(
    array(
        'name' => 'My Name',
        'pass' => 'my-password-clear'
    )
);

$opts = array('http' =>
    array(
        'method'  => 'POST',
        'header'  => 'Content-type: application/x-www-form-urlencoded',
        'content' => $postdata
    )
);

$context  = stream_context_create($opts);
$result = file_get_contents($url, false, $context);

It's odd because this code works fine with a simple form...