function getID() {
    if (session_status() == PHP_SESSION_NONE) {
        session_start();
    }
    if (isset($_SESSION['id'])) {
        return $_SESSION['id'];
    } elseif (isset($_COOKIE['id'])) {
        return  $_COOKIE['id'];
    } else {
        echo "Unexpected error.";
        die();
    }
}