As much as I hate to say it... depending on your cost/expense/budget thus far, your simplest solution might actually be to go the route of... Contribute (gags a little) but hey it's true.
You could "tag up" your template code relatively easily which would allow them to use Contribute to manage the static content inside your php scripts. Since Contribute is aware of PHP's presence, it should help prevent them from breaking the site by changing things in a text editor or other wysiwyg editor.
The other option I'd suggest would be to go the route of looking into something like TinyMCE editor and building a "wrapper" php script for editing. The wrapper would be given the page to edit, it would then load the page directly via the file system and allow the user to edit the "content blocks" in that page which could be specified via a delimiter/tag you use like .
Those are just a couple "quick fix" options to get you what you want with the minimum amount of rewriting/modification needed.