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!
http://www.dreamdealer.nl/articles/10_common_PHP_security_mistakes.html
Convert PHP Warnings and notices into fatal errors