If you're using an Apache server it's really quite simple, and doesn't require you to change any file extensions or links.
In your Apache httpd.conf file, look for the definition of the PHP MIME type and add the .html file extension to the space-separated list, i.e.:
Code:
AddType application/x-httpd-php .php .html
If there are additional definitions (i.e. for php4), add .html to those lines, as well.
Restart the web server to apply the change.
This will cause all files with the .html extension to be parsed by the PHP engine before delivery. You will now be able to include whatever PHP code that you like within those pages, and they will be handled just like the default .php files.
There are discussions about server overhead using this method, and I expect we may see some comments here about that, however on even high-load servers I have not seen any significant resource issues as a result of handling files in this manner. If your site is all .html instead of all .php, the PHP process is handling the same number of files that it normally would, anyway.