What would be the best (possible) way to create a download mechanism so that when a user clicks a link to download a file, the backend PHP script prompts the download window and the file downloads from a directory not in the web (http) root directory?

In this particular situation, I have a varying number of files (from 50MB to 1GB) on a seperate drive to the http server (Apache) root directory. I have come accross the PHP header function but am not sure if a) it is the correct way to go or b) that I am barking up the right tree!!

Any advice appreciated!