Class 'Mage' not found in Controller.php on line 835

Struggled fo serveral days, but not solved.

I get error when clicking magento connect manager in backend

PHP Fatal error: Class 'Mage' not found in "/abc/downloader/Maged/Controller.php on line 835"

The error also ocurred when just access http://domainname/downloader   The same code works for another host with OS Ubuntu.  The error host is CentOS release 6.5   It is not a privilege problem. I run chmod -R a+rwx * to change privilege of all folders in testing environment, but error still exist.

By tracing /abc/downloader/Maged/Controller.php on line 835

It is invoking Mage::setIsDownloader();

Before that line is include_once $this->getMageFilename();

Controller.php works. I add a line of output of the variable $this->getMageFilename();, it shows /abc/app/Mage.php"

And I write a test php and put it in downloader folder. It works:

echo "hi";
include_once('/abc/app/Mage.php');
 
echo Mage::getVersion();

The test php works. When accessed by http://.../downloader/test.php It can output   hi1.7.0.2 

I also tried to add include_once('/abc/app/Mage.php'); manually in index.php of downloader folder, but fails.

Cache and session are removed from both /abc/var and /tmp/magento

Any hint to further trace the problem?

via Chebli Mohamed