Extra server variables that are available to cloud flare are:
$_SERVER["HTTP_CF_CONNECTING_IP"] real visitor ip address, this is what you want
$_SERVER["HTTP_CF_IPCOUNTRY"] country of visitor
$_SERVER["HTTP_CF_RAY"]
$_SERVER["HTTP_CF_VISITOR"] this can help you know if its http or https
you can use it like this:
if (isset($_SERVER["HTTP_CF_CONNECTING_IP"])) {
$_SERVER['REMOTE_ADDR'] = $_SERVER["HTTP_CF_CONNECTING_IP"];
}
If you do this, and the validity of the visiting IP address is important, you might need to verify that the $_SERVER["REMOTE_ADDR"] contains an actual valid cloudflare IP address, because anyone can fake the header if he was able to connect directly to the server IP.