How do I get proxy ip address in codeigniter

[eluser]Unknown[/eluser]
I'm new to codeigniter. I'm developing a application in codeigniter I need to insert proxy ip address in database so how I get the proxy ip address in codeigniter. When I use $_SERVER['HTTP_X_FORWARDED_FOR'] its giving me error

Code:

A PHP Error was encountered
Severity: Notice
Message: Undefined index: HTTP_X_FORWARDED_FOR
Filename: controllers/filename.php
Line Number: 51

I search many forums for getting proxy ip in codeigniter but didn't find any solution to my related problem. Thanks in advance.