Metti il file esterno in una directory del tipo miosito.it/script/miofile.php
e in miofile ci scrivi
//import librerie joomla
define( '_JEXEC', 1 );
define('JPATH_BASE', "../");
define( 'DS', DIRECTORY_SEPARATOR );
require_once ( JPATH_BASE .DS.'includes'.DS.'defines.php' );
require_once ( JPATH_BASE .DS.'includes'.DS.'framework.php' );
require('../libraries/joomla/factory.php');
// initialize the application
$mainframe =& JFactory::getApplication('site');
$mainframe->initialise();
//get id current user
$user =& JFactory::getUser();
$id_current_user=$user->id;