Script to Compare Database Structures

I know I CAN hack something together but I hate to reinvent the wheel. I want to be able to compare the structure of two different clients databases that might be on different servers that are firewalled away from each other. Given the two structures it will list all the SQL commands needed to make the database structure the same. In a perfect world on one side you would pull up a PHP page that does a "generate structure" which would create a downloadable file which you could then upload to the other system which would then give a listing of the SQL commands needed to make the local structure match the uploaded structure. Thanks in advance... Matt