Would the subscription give the user access to certain features on the site? This is something that we've built before a few times using different methods.
One method is to use the Stripe subscription API to allow a user to sign up to certain monthly subscriptions, each month their subscription is automatically renewed until they cancel.
Another method is to use Simple Cart to create a one-off subscription product. When a user purchases the product we give them a subscription for a certain amount of time. Once that that has expired they have to purchase the product again. This method is much simpler for us to implement, but the subscription is not automatically renewed.
If this is something you'd be interested in us helping you with, you can send an email to email@example.com and I can give you more details.
PHP Programmer - interactivetools.com