Clarify includes for header and footer

Hi,

Could you clarify how to add a header and footer to formmailer.php?

I read the tutorial and have the manual, looked up on google server side includes.

My understanding is I can make includes a few ways;

an shtml file with the include statement pointing to a .html file of my header.

an shtml file with the include statement pointing to a .php file of my header.

or just a .html or .php file with my header or footer code.

It looks to me like your code in formmailer.php generates the include statement.

So do I just need to create a header.html file and footer.html file an point to those?
Or do they need the extension .php?

Thanks.