How to fake $_SERVER['REMOTE_ADDR'] variable?

I assume that you mean faking it remotely. The short answer is yes you can. The long answer about how easy it is depends on how you want to fake it.

If you don't care about receiving a response, it's as trivial as opening a raw socket to the destination and forging the source IP address. I'm not sure if it's really easy to do in PHP since all of PHP's socket implementations are at or above the TCP level. But I'm sure it's possible. Now, since you're not in control of the network, the response will not go back to you. So that means that you cannot (reliably anyway) create a TCP connection via a trivial forged TCP header (since the syn-ack does prevent this by requiring two-way communication).

However, if you can compromise the gateway the IP is off of, you can do whatever you'd like. So if you compromise the wifi router a computer is connected to, you can pretend to be that computer, and the server won't tell the difference. If you compromise the ISP's outbound router, you can (in theory at least) pretend to be the computer and the server won't tell the difference.

For some more info, see these following links:

• ServerFault Question
• Symantec Article
• Linux Security Article

However, you will only be able to forge the 127.0.0.1 loopback address under TCP if you actually compromise the local machine/server. And at that point does it really matter?

Important

If you're using a framework to access this information, be absolutely sure that it does not check the X-HTTP-FORWARDED-FOR header! Otherwise it's trivial to fake the IP address. For example, if you're using Zend Framework's Zend_Controller_Request_Http::getClientIp method, be absolutely sure that you pass false as the parameter! Otherwise someone just needs to send an HTTP header: X-Http-Forwarded-For: 127.0.0.1 and they now appear to be local! This is one case where using a framework without understanding how it works in the backend can really be bad...

Edit: Relevant

I wrote a blog post recently about how I stumbled across a vulnerability in StackOverflow's application. It's very relevant here, since it exploits a very similar mechanism to what this question is looking for (although the circumstances around it are somewhat narrow):

How I Hacked StackOverflow

The remote address is not something added out of courtesy, it's used in the IP protocol to route packages, so if you send a package with a fake address, you will not receive a response, and since you're talking about a HTTP request, which is delivered over a TCP connection, which takes several IP packets (and the matching responses) to set up:

No, that's impossible (except of course by actually sending the request from the same host via the loopback interface).