shtml or php ??

Which is the best language to use, php or shtml, I use only a couple of SSI commands in shtml to call in headers and footers, and run cgi scripts as below. I thought maybe as I am getting more into php that I should change the files from shtml to php, and use the equivalent commands to call in headers, footers and run external .cgi programs. But as below cant see how to load in an external .cgi program into php, as you can see below. I am presently using an shtml file using <!--#include file="my.htm"--> which work fine and in php I can use the include('my.htm'); command. in one of my shtml i use the <!--#exec cgi="my.cgi"--> command, but how do I run an external .cgi script from php, I have looked at many of the commands such as include(), system(), passthru() , but cant find away to do it. Can someone help ?? Is it best for me to leave it in shtml as I have also discovered that SSI will not parse inside an html section in php. Which of these shtml, or php uses more resources on the server ? Which is the best solution to my problem ?? Regards Dave