Question about directory & file operations

I have a script on my local test website that lists all of the files in my FTP server directories. It is setup to look at absolute paths. (ie: C:/Windows) If it is called with the right parameters or the "Previous Directory" link is clicked too many times, the browser will be outside of the paths that I want them to be in... For example, the browser can address http://localhost/script.php?path=C:/ and view the contents of the C: drive. Not really what I want.. I would like to be able to lock the browser down to a particular set of directories and thier subs. Does anyone have any suggestions that might help? Thanks in advance...