php gzfile无法正常工作

This is working fine: readgzfile($filename);

This is not working: gzfile($filename);

gzfile function outputs this error:

Warning: gzfile(http://www.example.com/abc.xml.gz): failed to open stream: HTTP request failed! HTTP/1.1 503 Service Unavailable.

I checked my php.ini and phpinfo and found nothing is disabled!

I have verified with if (function_exists('gzfile')) to make sure it's available. 503 or any other error isn't taking place for readgzfile function!

This is a Windows 2008 server and PHP is 5.3.23.

Thanks for your kind help.