Alternative to $_SERVER['REMOTE_ADDR'] in php

Question

Is there any alternative to the $_SERVER['REMOTE_ADDR']. Which returns the ip address of the computer accessing a site. I'm trying to search about same external ip assigned by router, and got into this: How do two computers connect to same external address through NAT? And found out that the same external ip is assigned if the computers are connected to the same modem. I'm creating a simple login program in php which uses $_SERVER['REMOTE_ADDR'] to determine if a user is already logged in somewhere else in the same network. And this won't actually work if those computers are connected to the same modem through the router.

Answer 1

No, this is the best you can do. The server only knows where the request is coming from, and that may be a proxy or a NAT router or some other entity which is not the direct enduser. There's nothing you can do about that, that's how networks work.

The solution is simple: Don't use IPs to identify users. Ever. Use cookies.