Peter,
I'm in exactly the same situation as you, and believe me..... it's caused a few headaches🙂 Although it's not in the topic of your question, my advise would be to try and abstract your PHP code from the FrontPage generated HTML as much as possible. If you start trying to embed your code within his pages, it's going to start causing problems when it comes to updating your code. You'd need to have a current version of his work and update all files using FrontPage..... ugly🙂 My solution to this problem was to write all my PHP code in separate files and then give my client the require() statement needed to call the PHP file, which then in turn called all the files it needs and does the computational work. Not exactly the most elegant of solutions, but it works🙂
The other thing you should be aware of is that although the FrontPage docs say that it catagorically ignores all non-HTML code and leaves it alone, FrontPage doesn't seem to recognize any tags other than <script> and <% as open script tags. Using <? or <?php as you normally would confuses FrontPage (I mean God, why would anyone choose to use anything besides ASP) and it interprets the code as HTML. Place all your PHP within:
<script language="php">
....
</script>
to avoid this problem. Best luck and sympathies from someone trying to deal with the same situation🙂
Cheers,
Geoff A. Virgo