All of PHP scripts above run on the same box (same hosting) and same domain.
First, I want to integrate all of them (user table) to have an one-time-login, but it is too hard and will be very difficult to add plugins / add-ons or update.
So, I think some code lines / snippet (in a single file .php as class / library to include). I will insert this file into drupal, phpbb, wordpress,... to check login status. All of script will be share a session or cookies and will active user login status of itself.
I don't want to use other solutions like LDAP, OpenID because of curve.