http://php.net/manual/en/session.configuration.php#ini.session.save-handler
If you choose the default files handler.
session save path
File: ckfinder/config.php.
// get the session_id from codeigniter cookie.
$session_id = $_COOKIE['ci_session'];
// get the session data by session_id,replace to your session save path
$content = file_get_contents(dirname(__FILE__) . '/../../application/common/ci_session/ci_session' . $session_id);
// Start new or resume existing session
session_start();
// Decodes session data from a session encoded string
session_decode($content);
If you choose the redis handler, you can do like below:
$session_id = $_COOKIE['ci_session'];
// $content = file_get_contents(dirname(__FILE__) . '/../../application/common/ci_session/ci_session' . $session_id);
$redis = new Redis();
$redis->connect('127.0.0.1', 6379);
$content = $redis->get('ci_session:' . $session_id);
$redis->close();
// session_id($session_id);
session_start();
session_decode($content);