Single Sign On across multiple domains [closed]

The SSO solution that I’ve implemented here works as follows:

1. There is a master domain, login.mydomain.example with the script master_login.php that manages the logins.
2. Each client domain has the script client_login.php
3. All the domains have a shared user session database.
4. When the client domain requires the user to be logged in, it redirects to the master domain (login.mydomain.example/master_login.php). If the user has not signed in to the master it requests authentication from the user (ie. display login page). After the user is authenticated it creates a session in a database. If the user is already authenticated it looks up their session id in the database.
5. The master domain returns to the client domain (client.mydomain.example/client_login.php) passing the session id.
6. The client domain creates a cookie storing the session id from the master. The client can find out the logged in user by querying the shared database using the session id.

Notes:

• The session id is a unique global identifier generated with algorithm from RFC 4122
• The master_login.php will only redirect to domains in its whitelist
• The master and clients can be in different top level domains. Eg. client1.abc.example, client2.xyz.example, login.mydomain.example