Error HTTP_X_FORWARDED

I have a problem when checkingout in my webshop. Website sends a “internal” ip ( 10.0.18.5) from isp and my payment gateway dont accept it.

My isp tells me to change this:
$_SERVER[‘REMOTE_ADDRESS’] ===> $_SERVER[‘HTTP_X_REAL_IP’]
(not working)

My payment gateway tell me to change:
return $tmp_ip;     ====> return $_SERVER[“HTTP_X_FORWARDED_FOR”];

My Code:

/**
     * Returns the clients IP address.
     *
     * @return string
     */
    public function getClientIP() {
        //Proxy handling.
        $tmp_ip = $_SERVER['REMOTE_ADDR'];
        $x_fwd = isset($_SERVER["HTTP_X_FORWARDED_FOR"]) ? $_SERVER["HTTP_X_FORWARDED_FOR"] : null;
        if(self::$x_forwarded_for && $x_fwd !== null) {
            //Cut out the first IP address
            if(($cpos = strpos($x_fwd, ',')) !== false) {
                $tmp_ip = substr($x_fwd, 0, $cpos);
                $x_fwd = substr($x_fwd, $cpos+2);
            }
            else { //Only one IP address
                $tmp_ip = $x_fwd;
                $x_fwd = null;
            }
        }
        $this->x_fwd = $x_fwd;

        return $tmp_ip;          
    }

HELP!!!!!!!!!!