Save user / temporary data on $_SESSION

Store data on session.

Example:

$session = new \Symfony\Component\HttpFoundation\Session\Session();
$session->set('var_name', $session->get('var_name',0) + 1);
drupal_set_message($session->get('var_name'));

This is similar to :
$_SESSION['mymodule']['test'] = "Just for Test";  
$data = $_SESSION['mymodule']['test'];

To save data on configuration database.

To set data:
    $session = \Drupal::service('user.private_tempstore')->get('mymodule');
    $session->set('test', "Just for Test");

To get data:
    $session = \Drupal::service('user.private_tempstore')->get('mymodule');
    $data = $session->get('test');