When you are trying to set or get $_SESSION variable in Joomla environment using standart approach from an external script, it appears to be empty or not set.
$_SESSION['variable_name'] = "test"; $test_var = $_SESSION['variable_name']; // $test_var is empty!?!
It happens because Joomla uses its own session handling with come unique session-id-generation and some encryption in place, so the only way to get into the Joomla session data is to use the appropriate Joomla functions. I recently had a project where I needed to make simple data-input form with captcha generator and the original captcha data was stored in (and then got from) session.