I went into more detail about what the different processes are of adding php to a website programmed with *.html pages in this earlier htaccess post.
Here are a few more .htaccess lines that will enable php on your server. (Different servers require a different line of code.)
AddType application/x-httpd-php .html .htm
AddType x-mapp-php5 .html .htm
AddHandler application/x-php .html .htm
AddHandler x-httpd-php5 .html .htm
AddHandler x-httpd-php .html .htm
Check out my other .htaccess posts.