For static HTML files (.htm, .html) there are two options.
  The first option is to rename each file from .htm (.html) to .php and follow the instructions.
  The other option is to enable Server Side Includes (SSI) for .htm (.html) files.

If you server supports Server Side Includes AND allows you to customize which files will be used.
You will need to enable SSI for your .htm (.html) files and then add the following code to your pages:
<!--#include file="/display.php" -->

*Note: The following is for the Apache Web Server and may (not) work with your webserver.
http://httpd.apache.org/docs/2.0/howto/ssi.html
The following is a slight modification from the Apache Web Server Documentation
Locate and modify the .htaccess file that is located in the root / directory of your website.
Add the following lines to the top of the file:

Options +Includes
AddType text/html .htm
AddType text/html .html
AddOutputFilter INCLUDES .htm
AddOutputFilter INCLUDES .html