how can your link session with two sites

if you want login in
www.gradtrain.com
and my session is also available in
gradtrain.com/forum
?php
define('IN_PHPBB', true);
define('ROOT_PATH', "forum/");

if (!defined('IN_PHPBB') || !defined('ROOT_PATH')) {
exit();
}

$phpEx = "php";
$phpbb_root_path = (defined('PHPBB_ROOT_PATH')) ? PHPBB_ROOT_PATH : ROOT_PATH . '/';
include($phpbb_root_path . 'common.' . $phpEx);

$user->session_begin();
$auth->acl($user->data);
?>
$auth->login('nanhe', 'nanhe', $remember, 1, 0)
?>
$user->session_kill();
$user->session_begin();
?>