I'm a bit stuck adding new features to m0n0wall.

I've downloaded the interface status PHP page, and uploaded it with a
different filename, and I get HTTP error 500 (server error) when I try to
visit the page.

Where do I begin writing my own pages? Is there a template?

And how do I fix getting error 500? I'm basically downloading the PHP file,
and copying its contents into a new one.