Build a Paywall for Digital Content

I built a website for a digital service for my field (chemistry). The site is built, the registration system is built, the functionality is built. I coded it all by hand in html, jquery, and php. That's not where I need the help. The customer can already register and activate a free 7-day trial. What I don't have is a way to get people to pay for it. I want a monthly/yearly subscription service, and PayPal seems a decent way to go. I understand this will involve the PayPal API, and this is one area of coding where I am not competent. What I need: *When the customer clicks 'buy now,' and their user profile is created, *they are directed to the PayPal checkout system. *They decide on monthly or yearly subscription and *pay. *PayPal redirects to my site and *their mysql database is updated from 'trial' version to 'full' version. *They then can login and have access to the digital service. *If they don't re-subscribe, they lose access. That's what I have in my head, at least. If there are better/more efficient ways of accomplishing this same task, I'm open to better suggestions. My server just needs to know they've paid so they can unlock the content, and know when they've unsubscribed so they can be blocked from the content. This is the last piece of the website before I launch publicly. I don't really expect it to be that complicated of a job, I just don't know how to do it. Ideally, I'd like the job done before 11am (US Eastern Time) Friday October 23.