By SSI, do you mean any type of server side includes or a certain type in particular? such as
<!--#include virtual="/cgi-bin/example.cgi?argument=value" -->
If you want that kind of server side include, it needs to be enabled. Here's a link that tells you how: http://httpd.apache.org/docs/mod/mod_include.html
if you just want to use includes, then php has an include() function.
include("path/to/file/filename.php");
more at http://www.php.net/include
Hope this helps.
Cgraz