Well, SHTML is basically just HTML with SSI.  SSI means Server-Side-Includes.
Therefore, all of your SERVER-SIDE SHTML files had lines in them including other files.  (usually more html)
You can change all the SHTML and HTML to PHP if you go thru all the pages and change the INCLUDE’s to
the PHP format.  Here are some examples:
include ‘homepage.php?foo=1&bar=2’;
include ‘www.myserver.com/photo.php’;
(the first has arguments, the second a domain name… And, assumes inside PHP tags…)
Please note that you must decode the page that the HTML include is loading from and make it the same inside the PHP line.  The PHP includes must be inside PHP tags or can be done like this:
<?PHP include'homepage.php'; ?>  (Which can be inside of anywhere in the standard html just like the SHTML)
Hope that makes sense!  Good luck…