Hello. I have oauth authorization from Patreon on my site. I use the PHP library for this and store access_token and patron_id of each user in the database. How can I now find out if a given user is a patron for my campaign? I need to do this without user intervention (cron).
access_tokens for users are not necessary for this. You can just use your creator access token (in the app details) to call the members endpoint for your campaign, iterate among the members and get their membership details.
If you get stuck on trying to get any particular field you need from the memberships endpoint, you can just post it in this forum.