For users who will be inputting php code, you should disable tinymce by default -- the setting can be adjusted in users' profiles, and sitewide via the tinymce settings.
It's a shame there's not an easy way to do that. It's very rare that any pages will need php code. I just have a few pages where I use <?php include("page.html"); ?> to include external pages.
Ah, well. Thanks for the info!
- jody