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?
This is not PHP, or Apache -- it is a security 'option' of whatever FOSS package you are using. PHP sessions have no inherent security. 

If *Joe* with IP 192.168.0.1 logs into your site, he might get a session token of **ABC123**. If *Jane* then goes to her workstation, manually creates the cookie that contains the session ID **ABC123** and then goes to <http://yoursite>, she will appear as if she is *Joe* to your system. This is critically important when users are using a proxy (users from the UAE for example are routed out via a series of 3 regional routers) that can change -- AOL is another example.

There should be a configuration option to turn this security feature off.