Get user IP in PHP

/*
 * returns IP on success, null on error
 */
function get_the_ip() {
    $ip = null;
    if ( ! empty( $_SERVER['HTTP_CLIENT_IP'] ) ) {
        $ip = $_SERVER['HTTP_CLIENT_IP'];
    } elseif ( ! empty( $_SERVER['HTTP_X_FORWARDED_FOR'] ) ) {
        $ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
    } else {
        $ip = $_SERVER['REMOTE_ADDR'];
    }
    return $ip;
}