Passing authentication from one site to another?

[roopurt18]

I wasn't sure if this was the best place for this thread, so please move it to the appropriate area if necessary.

 

I manage a site using a PHP / MySQL back end that tracks user authentication via sessions, nothing magical about that.

 

A design company is creating a new website for one of our clients and they'd like to have access to our user database so that the mutual client's users would be able to use the same user authentication on each site.  In addition, they'd like to provide the functionality that if a user logs into the new website and clicks the link to our website that they'd enter our website as already authenticated.

 

I'm reluctant to provide any outside access to our database, although in reality I'd only be granting "SELECT" access on a single table for a user that had to come from a specific host, which is easy enough.

 

It's the second part about passing authentication from one server to the next that confuses me.  Is this even possible?  I've thought about it a little and can think of one, maybe two ways in which it might be accomplished, but they both seem unlikely.

[metrostars]

The only thing I can think of is using cookies.

 

Never tried it, but read this:

 

http://www.jontodd.com/2006/08/10/php-session-cookie-multiple-domains/