$cart = array();
$cart['items'][] = array(
        'action' => $app->input->get('action', 'add', 'STRING'),
        'domain_name' => $app->input->get('chosen_domain', '', 'STRING'),
        'chosen_plan' => $app->input->get('chosen_plan', '', 'STRING'),
        'chosen_location' => $app->input->get('chosen_location', '', 'STRING'),
        'chosen_type' => $app->input->get('chosen_type', '', 'STRING'),
        'chosen_id' => $app->input->get('chosen_id', '', 'INT'),
        'chosen_period' => $app->input->get('chosen_period', '', 'INT')
    );
$session = JFactory::getSession();
$session->set('cart', $cart);
$cart_result = $session->get('cart');

echo "<pre>" . print_r($cart_result, TRUE) . "</pre>";