HTTP Header faking - OR: how to login to a page with PHP

How do I login to a (secured) login page via PHP? It's possible, apparently. What we need is to set up the correct HTTP request, by setting the header and field values.

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;
}

This function (found at
http://netevil.org/blog/2006/nov/http-post-from-php-without-curl)
gives me a PHP entry point. But how do we actually setup the header - what do we need to fake the Session to believe a User inputted some stuff in the login page?