static function sendXmlHttpResponseHeaders($contenttype = "application/json", $charset = 'utf-8')
	{
		if ( is_null($charset) )
			$charset = mb_internal_encoding();
			
		// charset
		header("Content-Type: $contenttype; charset=$charset");
		
		// no cache
		header("Expires: Sat, 1 Jan 2005 00:00:00 GMT");
		header("Last-Modified: ".gmdate( "D, d M Y H:i:s")." GMT");
		header("Cache-Control: no-cache, must-revalidate");
		header("Pragma: no-cache");	
	}