See the answer I've edited here:
Here is some sample code you can use within an Observer that listens
for the checkout_cart_product_add_after
or
checkout_cart_update_items_after
events. The code is logically the
same except checkout_cart_product_add_after
is called for only one
item and checkout_cart_update_items_after
is called for all items in
the cart. This code is separated/duplicated into 2 methods only as an
example.
For configurable products, you need to check for $item->getParentItem()
as in the sample code from that answer:
Event: checkout_cart_product_add_after
/**
* @param Varien_Event_Observer $observer
*/
public function applyDiscount(Varien_Event_Observer $observer)
{
/* @var $item Mage_Sales_Model_Quote_Item */
$item = $observer->getQuoteItem();
if ($item->getParentItem()) {
$item = $item->getParentItem();
}
// Discounted 25% off
$percentDiscount = 0.25;
// This makes sure the discount isn't applied over and over when refreshing
$specialPrice = $item->getOriginalPrice() - ($item->getOriginalPrice() * $percentDiscount);
// Make sure we don't have a negative
if ($specialPrice > 0) {
$item->setCustomPrice($specialPrice);
$item->setOriginalCustomPrice($specialPrice);
$item->getProduct()->setIsSuperMode(true);
}
}
Event: checkout_cart_update_items_after
/**
* @param Varien_Event_Observer $observer
*/
public function applyDiscounts(Varien_Event_Observer $observer)
{
foreach ($observer->getCart()->getQuote()->getAllVisibleItems() as $item /* @var $item Mage_Sales_Model_Quote_Item */) {
if ($item->getParentItem()) {
$item = $item->getParentItem();
}
// Discounted 25% off
$percentDiscount = 0.25;
// This makes sure the discount isn't applied over and over when refreshing
$specialPrice = $item->getOriginalPrice() - ($item->getOriginalPrice() * $percentDiscount);
// Make sure we don't have a negative
if ($specialPrice > 0) {
$item->setCustomPrice($specialPrice);
$item->setOriginalCustomPrice($specialPrice);
$item->getProduct()->setIsSuperMode(true);
}
}
}