natas6

This challenge has a view sourcecode button. Since PHP is a server-side language, unlike javascript we can’t see the sourcecode. The challenge devs give us snippets of it for challenges that require knowing how the website functions.

Looking at the code, we see a line with

include "includes/secret.inc";

going to natas6.../includes/secret.inc we see a secret FOEIUWGHFEEUHOFUOIU

Entering this secret gets us the flag!