The need to execute PHP in HTM or HTML files may occur for a number of reasons. The most common of which seems to be adding content managed portions to an existing HTML website. The .htm or .html pages will be part of the navigation, previously indexed by search engines, and likely linked to by other websites. Instead of changing the URL to .php for our updates we can convince the server to execute .htm or .html just as it would a .php file saving both the developer and customer from a number of headaches.
This can be done by adding the following lines to your .htaccess file.

If your web server is using PHP as an Apache Module:

AddType application/x-httpd-php .html .htm

For web servers funning PHP as CGI

AddHandler application/x-httpd-php .html .htm

If you happen to be running IIS this can still be done but it is a bit more difficult and a topic for another post on another day.