SSI doesn't work

Adv. Guestbook. - I have a problem. I want to use server side include on my site but I can't seem to figure out how it works. I can only manage to include the files in the same directory. I want to place a include statement in the header and futter so I don't have to go and update it all the time. How can I include a file that is not in the same directory as the the header and futter file. And so it points to no matter what directury the files are in. Or can I do it in PHP? Please tell me how to do this if you can. And I'm new to SSI and PHP
And is there a way to include a php file inside .shtml page? And is there a way to use include inside the regular .html pages because that's what the heeder and futter are in the adv. Guestbook. Thank you.

strip the header and footer.php template files, with the exception of the javascript.

open up index.php, addentry.php and comment.php and place your code like this.

you will place your footer include below the code

do the same to addentry.php and comment.php

after you do this, you will have to edit lib/comment.class.php

near line 272 you will have to edit it like this.

you need to do that, or when someone posts a comment to an entry, the script will complain about headers already sent.