php code in shtml
===

Okay, so I'm very new to website design, so please keep that in mind.
I've been working on a website (with file extension .shtml), and the time has come to add php code to it.  It's only a little code, but it doesn't work on the .shtml website; the php code ends up in the source code as a comment.  I was wondering if there was a way to make php code work on an shtml website.
Thanks.