update product quantity in magento 1.8

Question

how to update downloadable product quantity in shopping cart in magento thuough php

$_product = Mage::getModel("catalog/product")->load($p_id);      
$quote = Mage::getSingleton('checkout/session')->getQuote();      
$item = $quote->getItemByProduct($_product);           
$quote->updateItem($item->getId(), array( 'qty' => $p_qty)); getCart()->save(); 

Answer 1

Try this:

$quoteId = 1251; 
$productItem['qty'] = 2;
$productId =  17;
$product = Mage::getModel("catalog/product")->load($productId);
$quote   = Mage::getModel("sales/quote")->load($quoteId);
$quoteItem = $quote->getItemByProduct($product); 

if ($productItem['qty'] > 0) {
    $quoteItem->setQty($productItem['qty']);
}
try {
    $quote->collectTotals()->save();
} catch (Exception $e) {
   echo $e->getMessage();
}