On level 6, there is a curious "Input secret" form. I wonder what it does? Well, there is also a "View sourcecode" link, which will presumably show us the source code for that form. Then, we can try to anaylze whether it has any weaknesses we can take advantage of.
After examining the PHP code, we can see that it simply checks if the value you POSTed in the text field is the same as the secret, and if so, gives you the password for the next level. Instead of trying to guess the secret, I wonder if we can access that
includes/secret.inc file to see what the secret is.
Yes, we can. Now we can use that in the web form to get the password for the next level.
Make sure you configure your webserver to deny access to secret information.