升级Magento从1.4.2到1.6.0出现问题：Fatal error: Undefined class constant ‘XML_PATH_USE_CUSTOM_ADMIN_PATH’

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今天心血来潮，重新体验了下从1.4.2升级到1.6.0版本，结果和第一次完美过渡完全不一样，出现一个以前没有出现的错误：
Fatal error: Undefined class constant ‘XML_PATH_USE_CUSTOM_ADMIN_PATH’ in \app\code\core\Mage\Core\Controller\Varien\Router\Admin.php on line 123

现在正在找解决方法呢，具体原理还没有明白。

现在引用下几位大哥的解决办法：

After upgrading the Magento mostly the following error is occurred.

Fatal error: Undefined class constant ‘XML_PATH_USE_CUSTOM_ADMIN_URL’ in /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

class Mage_Adminhtml_Helper_Data extends Mage_Core_Helper_Abstract
{protected $_pageHelpUrl;

To

{
const ML_PATH_ADMINHTML_ROUTER_FRONTNAME=’admin/routers/adminhtml/args/frontName’;
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