Using .htaccess to make all .html pages to run as .php files?

Create a .htaccess file at the root of your website and add this line:

[Apache2 @ Ubuntu/Debian: use this directive]

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

Or, from comment below:

AddType application/x-httpd-php5 .html .htm

If your are running PHP as CGI (probably not the case), you should write instead:

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