在特定目录中的特定html页面上启用PHP

I have a Rackspace Cloud Sites account and I'm trying to enable PHP for a single html file in a specific directory under my root site folder.

I have a .htaccess file in my root folder. And my document root from using $_SERVER['DOCUMENT_ROOT'] is /mnt/stor1-wc2-dfw1/myaccnum/servernum/mydomain.com/web/content(I've obscured a few details but the basic structure is intact).

I then do the following (which I saw on another SO answer):

<Directory "/mnt/stor1-wc2-dfw1/myaccnum/servernum/mydomain.com/web/content/mydir">
  <Files "index.html">
    AddType application/x-httpd-php .html .htm
  </Files>
</Directory>

However, I'm getting error "500 Internal Server Error". If I take away <Directory> and <Files> everything works but for all files.

How can I fix this?!

You cannot add types per directory or location. However, you can manipulate the engine ini setting of PHP. Set it to off globally and enable it only for your index.html.