MyTrueHost 
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After upgrading the Magento mostly the following error is occurred.
Fatal error: Undefined class constant ‘XML_PATH_USE_CUSTOM_ADMIN_URL’ in /home/test123/public_html/app/code/core/Mage/Core/Controller/Varien/Router/Admin.php on line 123
Solution : To resolve the above error refer the following steps.
1st take backup for the file /app/code/core/Mage/Adminhtml/helper/Data.php
2nd) Change the following line in the file /app/code/core/Mage/Adminhtml/helper/Data.php
From
protected $_pageHelpUrl;class Mage_Adminhtml_Helper_Data extends Mage_Core_Helper_Abstract
{
To
const XML_PATH_ADMINHTML_ROUTER_FRONTNAME = ’admin/routers/adminhtml/args/frontName’;class Mage_Adminhtml_Helper_Data extends Mage_Core_Helper_Abstract
{
const XML_PATH_USE_CUSTOM_ADMIN_URL = ’default/admin/url/use_custom’;
const XML_PATH_CUSTOM_ADMIN_URL = ’default/admin/url/custom’;protected $_pageHelpUrl;
Save the file and delete the Magento cache from the /var/cache directory and now browse the domain
