I have three different websites that I run on three distinct top level domains. I would like to be able to have users log into any one of them and then be automatically logged into all three. As an additional requirement I'd like the user to be able to use my own login system or a third party system of their choice (eg. Facebook).
Can anyone point me to clear documentation on a "standard" way to do this? Everything I've read suggests trying to write my own solution would be a bad idea. SAML seems to one possible direction to go, so I tried playing with SimpleSamlPhp, but I haven't been able to wrap my head around that (better examples than what they have in their documentation might help).
So what is the best way to accomplish this?