Hey John,
This may be a server issue. If you answer the next couple of questions I'll see what I can help you with.
1) Does your server support PHP - I'm sure it does but I may need some more information on this.
2) If so can you create a file called Test.php and place the following Code inside of it:
<?php phpinfo(); ?>
What this does is provides information on what version you are using - and any special items/configurations that the server your hosting your site on has done. If this file does not work, then you do not have PHP installed.
(You can upload Test.php to any directory or location on your server).
If that is working - Please Post the GuestBook.php Code you are working with and I'll look over it for syntax errors.