I'm posting to add my agreement to what
incrediBILL said, in case your admin remains skeptical until there are multiple replies.
The main reason being that I don't want to rename all the pages to shtml or php, to avoid broken links/awkward re-directs and general confusion.
Yes, good strategy.
rocknbil, I believe what the OP meant was awkward redirects
if they renamed .htm pages to .php. The renaming is the thing to avoid.
I'd still go with PHP includes over SSI, because it makes available the other features of PHP, which many people eventually decide they want, anyway. If you've converted to SSI, then you'd need a 2nd conversion, from SSI to PHP.
PHP as CGI is said to be slower, and theoretically should be, but on a shared server it does allow you to be much better walled off and protected from your neighboring sites on the server. Folders and files that need world-writable permissions (777/666) under Apache module can be made writable only by the owner (755/644) with CGI. In a shared environment, that is a worthwhile tradeoff.
I've used both module and CGI and although CGI probably is slower, it was nothing that I could notice as a user.
CGI is slower because the PHP interpreter, instead of being continuously loaded in memory and ready for use as a module, must be invoked from disk each time a .php page is to be processed.
However, the difference could be completely negligible if the server caches its most recently used disk files to memory for faster re-access. Windows does that as a matter of course, and I suspect Linux does it, too. In that case, the PHP interpreter would just be getting loaded from memory (at least most of the time), whether PHP is a module or a CGI.