So, you mean you want your PHP files to have a .html extension instead of a .php extension, correct?
That's pretty simple.
If you are running Apache just add this line:
AddType application/x-httpd-php .html
That will parse ALL of your .html files as php files.
Generally, the above line is defaulted to:
AddType application/x-httpd-php .php .php4
or something similar.
-- Jason