How to include PHP files in .shtml files?

Hi

Anyone know how to include PHP files in .shtml files? The code I am using is:

<!--#include virtual="/docs/includes/side_testimonials_wp.php" -->

At the moment I get this error:

[an error occurred while processing this directive]

When I follow the code on this page for .htaccess http://stackoverflow.com/questions/839272/how-to-include-php-file-in-sht... then it just downloads the file in the browser instead, and not renders it. This is the code the site and other sites recommend:

<Files *.shtml>
ForceType application/x-httpd-php
</Files>

There are similar other .htaccess file codes which either have no effect or result in the .shtml file being downloaded as a file to my computer.

Thanks