To setup a Linux server running Apache web server to process .html (.htm) files as PHP. create an .htaccess file in the root folder of your website and add these lines to the .htaccess file:

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

Source: http://www.lgr.ca/blog/2007/02/process-html-as-php.html