Previously I posted about a gentle introduction to php.
To make your .htm or .html pages process like they were .php, just add this line to your .htaccess file:
AddType application/x-httpd-php .html .php .htm
Or use:
RewriteEngine On
RewriteRule ^index.html /index.php
The .htaccess directory-level configuration file can also be used for:
Custom Error Pages
Password protection
Redirects
Prevent hotlinking of images
Preventing Directory Listing