如果没有设置，设置为零，然后总是加一.

If it's not set, set it to zero, then always add one.

if (!empty($_getvars['id'])) {
    $data = $session->get('cart');
    if(!isset($data[$_getvars['id']]){
        $data[$_getvars['id']] = 0;
    }
    $data[$_getvars['id']] += 1;
    $session->set('cart', $data);
} 

或者你可以添加一个动态数量

Or you could add a dynamic quantity

if (!empty($_getvars['id'])) {
    $data = $session->get('cart');
    if(!isset($data[$_getvars['id']]){
        $data[$_getvars['id']] = 0;
    }
    // $_GET['qty'] OR 1, if not set
    $qty = (!empty($_getvars['qty']))? $_getvars['qty']: 1;
    $data[$_getvars['id']] += $qty;
    $session->set('cart', $data);
}