It's easy to send custom information to $_SERVER[REMOTE_ADDR]; or $_SERVER[HTTP_X_FORWARDED_FOR]; ? I wish to make these vars output my custom text. I figured about editing headers sent by browser or delivering it with php script/curl program. But I'm not sure how. Please let me know if it's possible and just how.

You cannot change $_SERVER['REMOTE_ADDR'] as this is the Ip from the TCP connection. The only method to change that's to really connect from the different real Ip.

You are able to send whatever X-FORWARDED-FOR header you would like within the HTTP request. Everything header can be used for would be to hint the hooking up IP is proxying a request another client. It isn't depended upon for anything important.