Never include files from user input

Creating a navigation seems easy when you do this:

<html>
<a href="index.php?page=test">test</a>
</html>

And then include the page like this:

<?php
include($_GET['page'].'.php');
?>

This is really dangerous. Hackers can use this to display/include/run every file on your server, and even on other servers!

Convert PHP Warnings and notices into fatal errors