使用file_get_contents在PHP中发布方法？

I need to submit a form to a website (http://www.itu.int/online/mms/mars/ship_search.sh) and I use the following code:

$postdata = http_build_query(
    array(
        'sh_mmsi' => '246709000'
    )
);
$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('http://www.itu.int/online/mms/mars/ship_search.sh', false, $context);

For some reason is not triggering the search query. If I hit the Submit button in the URL address http://www.itu.int/online/mms/mars/ship_search.sh?sh_mmsi=246709000, then the results shows at the end. I wonder if the problem is that this website has two forms and I would need to specify which one to post to? If so, how should I do it?