Please remember that you have to start your session before use of $_SESSION becomes available. In most cases it can be done by uncommenting the "session_start()" call in the configuration file:
Once enabled, session variables can be accessed using the global $_SESSION array:
$_SESSION['CKFinder_UserRole'] = "admin";
Please do not call session_start() in the CheckAuthentication() function body, it is recommended to call session_start() at the beginning of config.php instead.
More information about session handling can be found in PHP manual.