I have three websites: a.example.com, b.example.com, and c.net. Each was built a couple years after the other and all have their own users table.
I want to integrate these sites together so that I can login with a single username/password combo and have access to all three sites just by clicking links without having to login again.
I've never done an integration like this before, so looking at two angles:
1) Create another website, 'd.net' where new accounts can be created. If you login to one of the original three sites, it asks you for your d.net account. If you dont have one it asks you to make one. In your 'account settings' on d.net, you can 'add sites' to your account by entering your old username and password for them. Sound reasonable?
2) Solving the technical problem of being able to one-click login to another of the three sites if i'm already logged into d.net. Afaik, cookies with a session ID can't be shared across domains. So... maybe generate a token and save it to the database. Send token over GET to another website, which can check the token, log the user in, and then delete the token. Sound about right?