PHP include menu.php

I have a menu.php where there are the include of css and js library. I want to create a new file inside a folder admin and "include menu.php". How can I do?

menu.php

<!-- BOOTSTRAP -->
<link rel="stylesheet" href="./css/bootstrap.min.css" >
<link rel="stylesheet" href="./css/timeline.css">
<script src="./js/jquery-3.3.1.slim.min.js"></script>
<script src="./js/popper.min.js"></script>
<script src="./js/bootstrap.min.js"></script>

<!-- DATETIMEPICKER -->
<script type="text/javascript" src="./js/jquery.min.js"></script>
<script type="text/javascript" src="./js/moment.min.js"></script>

Hierarchy

menu.php
-css
-js
-admin -> newfile.php