How to do PHP and MYSQL credentials

I'm developing an event system that will consist of a web system and a mobile application. One of the main points will be the accreditation (check-in and check-out) of the users in the lectures. After registering on the website and registering for the event, on the arrival of this user to the place of the lecture I intend to confirm his presence in the lecture in some way. I need some idea how to do this. Generate a bar code for each event and read it on the user's badge when it arrives at the event? Use QRCode and the user's cell phone? Does each event have a 'code' ID? So how do you prevent users who have not been present to have the presence confirmed by other people? Suggestions please.