根据访问国家/地区的客户设置商店货币

Question

I want to set My store Currency Based on the customer country.

<?php
   $ip = $_SERVER['REMOTE_ADDR'];
   $ipdat = @json_decode(file_get_contents("http://www.geoplugin.net/json.gp?ip=" . $ip));
   echo $ipdat->geoplugin_countryCode; // Here i am getting the country code

   if($ipdat->geoplugin_countryCode == 'US'){
       Mage::app()->getStore()->setCurrentCurrencyCode('USD'); // i tried this 
   }
?>

Mage::app()->getStore()->setCurrentCurrencyCode('USD');

I tried this but it is giving the following error.

Why is it giving an error ?

any Idea?