Transferring rows between 2 databases on 2 web servers

Hi Everybody,

There are n number of web servers all having the same Linux-Apache-MySQL-Php
setup, with identical databases. Each of these servers functions as a
billing server at a store of my company. Now I am required to set up a
master server that will copy all data from each of the store servers. All I
wish to know is:

How can a page on the master server obtain data from a store database by
calling a page located on the store server?

I do not wish to access a store database directly, but through a page on the
store server. Specifically, how can a web page get data from another web
page on another server.

Many thanks,
Ferdi