HTTP_X_FORWARDED_FOR properly done

HTTP_X_FORWARDED_FOR_Properly_Done.php

<?php

/*
  In the light of the latest httpoxy (https://httpoxy.org/) vulnerabilities, 
  there is really a need for a full example, how to use HTTP_X_FORWARDED_FOR properly.
  So here is an example written in PHP, how to detect a client IP address, if you know 
  that client may be behind a proxy and you know this proxy can be trusted. 
  If you don't known any trusted proxies, just use REMOTE_ADDR
*/

function get_client_ip ()
{
    // Nothing to do without any reliable information
    if (!isset ($_SERVER['REMOTE_ADDR'])) {
        return NULL;
    }

    // Header that is used by the trusted proxy to refer to
    // the original IP
    $proxy_header = "HTTP_X_FORWARDED_FOR";

    // List of all the proxies that are known to handle 'proxy_header'
    // in known, safe manner
    $trusted_proxies = array ("2001:db8::1", "192.168.50.1");

    if (in_array ($_SERVER['REMOTE_ADDR'], $trusted_proxies)) {

        // Get the IP address of the client behind trusted proxy
        if (array_key_exists ($proxy_header, $_SERVER)) {

            // Header can contain multiple IP-s of proxies that are passed through.
            // Only the IP added by the last proxy (last IP in the list) can be trusted.
            $proxy_list = explode (",", $_SERVER[$proxy_header]);
            $client_ip = trim (end ($proxy_list));

            // Validate just in case
            if (filter_var ($client_ip, FILTER_VALIDATE_IP)) {
                return $client_ip;
            } else {
                // Validation failed - beat the guy who configured the proxy or
                // the guy who created the trusted proxy list?
                // TODO: some error handling to notify about the need of punishment
            }
        }
    }

    // In all other cases, REMOTE_ADDR is the ONLY IP we can trust.
    return $_SERVER['REMOTE_ADDR'];
}

print get_client_ip ();

?>