In Apache .shtml are parsed by the server and it is no problem including any other file that the server can parse. You just have to use this SSI statement: <!--#include virtual="anyfile.php"-->
If it is not supposed to be parsed (because it is a plain HTML file or something) include file can be used instead, which is a bit faster.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.