edorsan on "users on different databases shared on a same root, for single login - register"

Dear bemdesign, thanks a lot for your reply :)
And yes, as you mentioned: I also do think that a custom solution will be necessary. A friend of mine is database expert, and he told me that he could run a database server at the backend, so that we could be able to control all of that. I will give it a try :) With the hope that he's good enough for not ruining my databases :D :D :D Thanks to all of you dear friends ! I will let you know, if there's success. Take care and have a wonderful day!