PHP Developer needed to create shared login across WordPress, XenForo and Piwigo Platform

Our three platforms:
WordPress, XenForo and Piwigo
Are developed in a way to allow creative sharing.

We require users that login to one to be automatically logged into the other, and wherever they log out to log them out of the other two.

Existing users should be merged into the shared user database.

WordPress is our top level site so this would be the best user manager, however it is preferred if users can have separate profiles on all sites, all independently manageable only shared between the platforms.

We do not wish to loose any of the functionality of any of the sites, i.e. XenForo has a sign in box that drops down, WordPress has a Java sign in that drops down also, we wish to have the current sign in areas edited to handle requests from where the new users are stored.

At very least we could have a script that connects the toplevel (WordPress) account to the others creating a one click registration and connected accounts to XenForo and Piwigo.