[hiding download source location]

hello all:

i am trying to write a system that will hide the location of a file download so people can not download the files without being logged into the system.  i found a tutorial to do this on phpfreaks a few years ago, but i tried looking today and was not able to find anything, but that could easily have been because it is under a different name now.

what i'm looking for is something that shows the link as (domain_name)/download.php?id=859, and when someone clicks on the link, the regular "file download" pops up asking the user where they would like to save the file to.

does anyone know where i might be able to find something like this?  i would prefer a tutorial as i have to do a number of other things in addition to this.

any help is greatly apprecaited.

thanks.