Server Side Includes can be called using different commands:
TEXT, HTML, SHTML OR CGI
CGI
Now here is the thing about SSI.. your server can be VERY picky on how you have to type out the command.
Some servers will take it as is:
some will error and you must type it out like this:
Whatever the case may be, it takes it from the ROOT directory, so you HAVE to make sure that you are typing it out from the root directory and not simply placing it in the same folder.
Both ASP and PHP cannot be executed using SSI because SSI requires the .shtml or .shm extension on your file and since both pages are 'executable' type pages, they are cannot simply run on virtual include tags.. OR exec cgi tags. Both ASP and PHP is what CGI and HTML is combined (the use of variables to execute an entire page).