You're already using .php on that test page. Since PHP doesn't understand the <!--#include --> command, you'll need to switch to the <?php include() ?> function.
There's probably more than one way to configure Apache, but if you want to have PHP code parsed inside of .shtml, what you found won't work. That would be to configure Apache to parse for SSI includes inside of PHP pages.
EPC Tutorials... We're here to help!
Offering custom integration services. Contact me here
* Not affiliated with EasyPHPCalendar or NashTech Inc.