i have 2 sites (example.com, ex2.com). Fisical is a 1 site with 1 db. When user sing in ex2.com, he was sing in example.com too. How do this?
P.S. Can do this with ZF?
I found a very interesting article on this topic. The author gives some ideas how to implement Multidomain authentication. http://codeutopia.net/blog/2008/09/25/sharing-authentication-over-multiple-sites-single-sign-on/
You could take a look into something like http://cosign.sourceforge.net/. Cosign enables you to create a single point of login for multiple sites. It's not specific to Zend but should work.
Have you looked into OpenID? You could lock OpenID consumer "ex2" to allow logins only from your "example.com" OpenID provider.
I am also digging the subject. Some time ago, I already did my own implementation and got it terribly wrong.
At the moment I am wondering whether to setup an own OpenID provider and locking consumer sites to accept only it. Another alternative would be yet another own implementation with a CAS style setup, where only a hash is passed via the browser, and the user verification is done server-to-server in the background using the hash as a disposable key.
I am not yet sure which one to pick or would some third alternative be better.