Store emulation on Magento <http://jacques.sh/2013/11/et-si-on-emulait.html>

bad.php

<?php
// Get the old store
$oldStore = Mage::app()->getStore(true);

// Set the new store
/* @var $customer Mage_Customer_Model_Customer */
$newStore = Mage::app()->getStore($customer->getStoreId());
Mage::app()->setCurrentStore($newStore);

// Here my code... Am I really on the new store?

// Set the old store
Mage::app()->setCurrentStore($oldStore);

good.php

<?php
// Start emulation
/* @var $customer Mage_Customer_Model_Customer */
/* @var $emulator Mage_Core_Model_App_Emulation */
$emulator = Mage::getModel('core/app_emulation');
$initial = $emulator->startEnvironmentEmulation($customer->getStoreId());

// Here my code... Yes! I'm really on the new store!

// Stop emulation
$emulator->stopEnvironmentEmulation($initial);