it's possible in plain PHP (provided that you are allowed to use fopen to write files, which is not possible on every server).
I suggest you take a look at the associated pages in the php-manuel (e.g. http://www.php.net/fopen ), then repost more detailed questions.
Also, you may (or may not) find a little script i posted earlier today on this board of interest. The link is http://www.phpbuilder.com/board/showthread.php?s=&threadid=10210163
it could be adapted for a simple guestbook script.
Just post any questions that arise 😉