How to include PHP files in .shtml files?


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 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.