Stopping users to see uploaded files

Dear friends, we are using php for developing a small tool for uploading files for download (publications). Using the usual move_uploaded_file, the publications are loaded into a directory where the webserver can write / read. Unfortunately, that means every person who connects to the directory can see the files. If some of the files are for sale, how do you stop the user from seeing them / downloading them without permission? Is there any other way to upload files using php? Thanks in advance! -- symbulos - ethical services for your organisation website www.symbulos.com