Deploying paid-for content

One of my existing clients who does face-to-face training wants to record training courses and sell them via their website. They want users to have time limited access to the courses to watch online, but to be unable to download them.

I was wondering what other freelancers have done to achieve this. I could write a system in PHP/MySQL to create time expiring licences, but I am wondering if there are any existing deployment systems available.

I don't want to re-invent the wheel if I don't have to!

Ray