Custom subscriber page. Nothing I do is working!

Just installed phpList. All went fine. But now I am trying to customize my public pages and nothing is working!

I am changing the header and footer. When I just add “Hello World” as text it shows up.
But when I am add has PHP nothing happens. I am not talking about the “settings” update. I am referring to the subscribe page settings.

It may just be me, but the manual instructions for this one topic are not very clear.

Any help appreciated.