How do file hosters count traffic?

[Momoe]

Just a heads up, the OP wants to know HOW they do it, what code/script whaever they use, because maybe he is working on something similar.

Yes right. I'm just interested in the technical aspect (don't want to actually use it for now).

Im guessing they use a system which callsback to them once the download its completed, try downloading a filesharing script and try to look and test out how it works,

I doubt I will find something since most if not all scripts are crap anyway. Of course you can implement it using php for example, but that is not efficient at all. I think the web-server has to be involved somehow as noted.