duplicating databases

I am running PHP/MySQL for a program that I am writing. We will have 100's or 1000's of databases that will be duplicates in structure. The problem is when I make a change to the database, I have to go to every database manually and make the change. All of the databases start with "a_" and are on the same server. I would like to be able to have one master database and then run a script when I make a change to it that will get the structure of the master database and check all of the other databases to make sure that their structures match. If not, it will make the change to the other databases. Does anybody know if there is all ready a program out there that would do this? Can anybody point me in the right direction? Or if it is only a few lines of script that somebody all ready has to do this, can you share it? Thank You Steve