Exams Are part of this site. Implemented with PHP and Symfony.
I was inspired to make this project after I obtained PHP Zend certificate.
In a Zend PHP certificate exam I get questions and I had to choose an answer of prepared test-like list. Some questions needed to input free-text answer by hand.
I wondered how difficult would be to implement such questions with answers system.
Btw the W3CSchool also has similar test-style exams. The main difference is that W3CSchools provides correct and wrong answers list after you finish exam. Zend never does that.
When I planned the system architecture, one of the challenge was how to store the exam questions so that user can wall back and forth between questions if he wants to make a correction. So I made a decision to make a list of selected questions for the exam and store it to the database the moment the exam is initialized. When user answers a questions, the selection is stored in the selected question object (row).
Also the payment system is implemented in this project. If user wants to see correct answers, he must pay 1, 2, 3 or 4 Euros, depending on the exam size. Without that he sees only the summary score of the exam.
Payment provider is Stripe
Because exams are cheep and user might not want to pay 1 euro each time, the balance system is implemented, where user has a balance amount in his account which he may use in his exams.