I'm thinking about a good solution for implementing a sign up/login system that works across different domains and servers. A working example is Olx (you can register in one domain, and your login will work in the rest of domains).

The scenario is that every domain (one per country) has its own database. And there will be 2 servers (for example), each one will have the 50% of the domains (and so the 50% of databases).

What would you suggest to start with?

Database: MySQL 5.1 Server-side language: PHP 5.3

(I will be using Symfony 1.4, so if someone has some suggestion for this framework, it will be interesting too, although it is optional)