In iWeb add a page called Guestbook.
Publish and you will find a page called Guestbook.html
Replace it with the Guestbook.php one
After you published open the index.xml file in the site's folder which is responsible for the menu and where all the pages of the menu are listed in TextWrangler and change Guestbook.html to Guestbook.php
I can't try it since I currently have no computer but it should work.
If it doesn't work you would have to create your own menu linking to the .php but try the other way first.
No need for iFrames.
Unfortunately had no luck with this method. But, I have found this site http://www.wyodor.net/Ajax/Ajax.html
which indicates some steps. The problem is that I can't figure out the right steps for using php. It is not that well explained, because the are 2 placeholders text boxes which have different use and it is not clear which one should be modified.
Any idea of what is he trying to say?