$app = JFactory::getApplication();
$input = $app->input;
$itemid = $input->get('itemid');
$quan = $input->get('quan');
$session = JFactory::getSession();
$cart = $session->get('cart', array());
$cart['items'][] = array('itemid' => $itemid, 'quan' => $quan);
$session->set('cart', $cart);

if($itemid == 'itemid') {
    $cart['items'][] = array('quan' => 'quan'+$quan);
} else {
    $cart['items'][] = array('itemid' => $itemid, 'quan' => $quan);
}