I'm not sure if this will work, but my first suggestion would be:
Is PHP compiled as a CGI version?
By default, PHP compiles as a module, that cannot be called by the commandline.
Maybe it will work if you create a CGI version of the PHP program.
and call it like this:
<!--#PHP sub="sdfsdf"-->
Again, I'm not sure at all.
If you search the forum some more you'll find other methods of including PHP in HTML without using the .php exgtention.