There are various PHP script that can allow you to execute the function "To download, you must subscribe to YouTube channel". The most common script with this function is the API function. It allows users to have interaction with YouTube in form of subscription.
Why don't you look for a widget? And embed it to your web site? Any ways... You'll have to communicate with the server and YouTube.. Or else, the user will not subscribe and still be able to click next... In other words... It needs a authorization key for PHP / MySQL