Trying to make a start on using EventSub (PHP)

So not really making much progress, I’ve made changes according to the EventSub instructions rather than webhooks, but I’m getting a 401 unauthorized error in response. Am I correct that the Client ID in this instance is for the app and not for the user? And if it’s the latter, do they expire? We have one in the database, but that gives me the same “Must provide valid app token” response. I just want to check before I try and get a fresh token and break everything in the process!