function getUserIdByEmail($email) {
        $stmt = $this->conn->prepare("SELECT id FROM user WHERE email = ?");
        $stmt->bind_param("s", $email);
        if ($stmt->execute()) {
            // $user = $stmt->get_result()->fetch_assoc();
            $stmt->bind_result($id);
            $stmt->fetch();
            $userid = $id;
            $stmt->close();
            return $userid;
        } else {
            return NULL;
        }
    }