Yes. Thanks. I understand that the extension needs to remain .html as the
content of the file is outputted html and not server side php code to be
processed. 

And yes, if you add a mime type of php=text/html is does create the file name
with a .php extension however this is not what I am asking.

I am asking whether the file name can include the original extension in the
file name (using the custom defined -N option for example). My unique
situation requires me to know (ideally in the file name) what was the
extension of the original file (e.g. something like SomeFile-php.html could
tell me that the original file was named SomeFile.php even though the final
file is actually a .html file). 

One way to achieve this is to look in the comments in the source of the actual
file (e.g. <!-- Mirrored from www.somesite.com/SomeFile.php by HTTrack Website
Copier/3.x [XR&CO'2010], Fri, 25 Feb 2011 13:39:00 GMT -->) but I was
wondering whether there is a way to include the file extension in the file
name itself?
Thanks.