Take a look at this thread it deals with what you want to do I think: http://www.webmasterworld.com/php/3164561.htm
The last entry might be of help (reformatted by me):
function do_post_request($url, $data, $optional_headers = null) {
$params = array('http' => array(
'method' => 'post',
'content' => $data
));
if ($optional_headers!== null)
$params['http']['header'] = $optional_headers;
$ctx = stream_context_create($params);
$fp = @fopen($url, 'rb', false, $ctx);
if (!$fp)
throw new Exception("Problem with $url, $php_errormsg");
$response = @stream_get_contents($fp);
if ($response === false)
throw new Exception("Problem reading data from $url, $php_errormsg");
return $response;
}
Basically the solution is to make use of the built-in php stream handling for urls.