Ho risolto cosi
// Set flag that this is a parent file
define( '_JEXEC', 1 );
define('JPATH_BASE', dirname(__FILE__) );
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();
$user =& JFactory::getUser();
echo 'Ciao '.$user->username;
echo 'Ciao '.$user->email;