Question: My website is in PHP, running on Apache.

One of my users is on a WAN with 2 IPs and his connection gets routed to our server by any one of them.

PHP seems to log out the user out, if it detects change in IP.

It is an open source app and I think some common popular file must have been used.

Any way to prevent it?

Answer: 
I don't think apache has anything to do with that. The problem is most likely in php session although I don't think php checks the client ip by default. Are you sure there's nothing in your code checking the ip for a session?
I think it would be in PHP, not Apache. You might want to look see if the REMOTE\_ADDR is being used with $\_SERVER in generating the sessions. See [this PHP reference](http://us3.php.net/manual/en/reserved.variables.server.php), and [here is a whole thread](http://www.daniweb.com/forums/thread49898.html#) on the topic. 

You might have better luck on stackoverflow.