Hi, i am having problem understanding the wiki developer's api tutorial. i followed the first real world example and it works perfectly fine. what i don't understand is "$this->attach($this, array('NOTIFIER_CART_ADD_CART_END'))" why is that the "$this" is attached to the event and in the real world example 1 is attached in a $_SESSION['class']. can anyone explain to me why. and also i have experimented in the real world example 1. i removed the condition "if ($_SESSION['cart']->show_total() >= $this->freeAmount && !$_SESSION['cart']->in_cart($this->freeProductID) )
{

}" and when i tried to add a product in the cart an error occur. the error is INTERNAL SERVER ERROR 500. can anyone tell me why is that happening it is just a statement why is an error comes up.