PHP - Protecting digital Downloads

I'm trying figure out how I can protect digital downloads in PHP. Just need some general directions so I can start my research. I don't seem to be able to find anything useful.

I want to make files available for my users to download but don't want them to be able to directly access a download folder. Also, I want the download link to be available only for set period of time or a single download.

Could some one point me in the right direction?