Question

I am trying to remove just one qty from my cart rather than all, but to no avail.

Can anybody help?

Here is the code I have got so far…

require_once 'app/Mage.php';
Mage::app("default");
Mage::getSingleton("core/session", array("name" => "frontend"));
$session = Mage::getSingleton("customer/session");

$yourProId = $_POST['prodID'];
$qty = 1;

foreach (Mage::getSingleton('checkout/session')->getQuote()->getItemsCollection() as $item) {
    if ($yourProId == $item->getProductId()) {
        Mage::getSingleton('checkout/cart')->removeItem($item->getId())->save();
    }
}

UPDATE: Here is the code that works, thanks to R.S. for this!

$yourProId = $_POST['prodID']; 
$qty=1; 

$cartHelper = Mage::helper('checkout/cart');
$items = $cartHelper->getCart()->getItems();
foreach ($items as $item) {
if ($item->getProduct()->getId() == $yourProId) {
    $qty = $item->getQty() - 1; // check if greater then 0 or set it to what you want
    if($qty == 0) {
        Mage::getSingleton('checkout/cart')->removeItem($item->getId());    
    } else {
            $item->setQty($qty);
        }
        $cartHelper->getCart()->save();
        break;
    }
}

Answer 1

Try

$cartHelper = Mage::helper('checkout/cart');
$items = $cartHelper->getCart()->getItems();
foreach ($items as $item) {
    if ($item->getProduct()->getId() == $yourProId) {
        if( $item->getQty() == 1 ){
            $cartHelper->getCart()->removeItem($item->getItemId())->save();
        }
        else if($item->getQty() > 1){
            $item->setQty($item->getQty() - 1)
            $cartHelper->getCart()->save();
        }
        break;
    }
}

Take a look @ /app/code/core/Mage/Checkout/controllers/CartController.php

See http://www.magentocommerce.com/boards/viewthread/30113/