Hjdcti

PHP Shipping Estimation script (From stackexchange )

<?php
//require_once("Mage.php");
require_once(__DIR__ . '/app/Mage.php');
umask(0);
ini_set('display_errors',true); 

function getShippingEstimate($productId,$productQty,$countryId,$postcode ) 

 $quote = Mage::getModel('sales/quote')->setStoreId(Mage::app()->getStore('default')->getId());
 $_product = Mage::getModel('catalog/product')->load($productId);

 $_product->getStockItem()->setUseConfigManageStock(false);
 $_product->getStockItem()->setManageStock(false);

 $quote->addProduct($_product, $productQty);
 $quote->getShippingAddress()->setCountryId($countryId)->setPostcode($postcode); 
 $quote->getShippingAddress()->collectTotals();
 $quote->getShippingAddress()->setCollectShippingRates(true);
 $quote->getShippingAddress()->collectShippingRates();

 $_rates = $quote->getShippingAddress()->getShippingRatesCollection();

 $shippingRates = array();
 foreach ($_rates as $_rate):
 if($_rate->getPrice() > 0) 
 $shippingRates[] = array("Title" => $_rate->getMethodTitle(), "Price" => $_rate->getPrice());
 
 endforeach;

 return $shippingRates;


echo "<pre>";
//product id, quantity, country, postcode
print_r(getShippingEstimate('14419','1',"IN","642001"));
echo "</pre>";

?>

Error :

Fatal error: Uncaught Error: Call to a member function getModelInstance() on null in /home/abc/public_html/app/Mage.php:463
Stack trace:
#0 /home/abc/public_html/sp_cost.php(12): Mage::getModel('sales/quote')
#1 /home/abc/public_html/sp_cost.php(38): getShippingEstimate('14419', '1', 'IN', '642001')
#2 main
 thrown in /home/abc/public_html/app/Mage.php on line 463

How to solve the error and get estimation shipping charge?