shell_exec is extremely useful as a substitute for the virtual() function where unavailable (Microsoft IIS for example). All you have to do is remove the content type string sent in the header:
<? 
$mstrng = shell_exec('yourcgiscript.cgi');
$mstrng = ereg_replace( "Content-type: text/html", "", $mstrng );
echo $mstrng;
?>
This works fine for me as a substitute for SSI or the virtual() func.
Anton Babadjanov
http://www.vbcn.com.ar