share data between server applications

I have 3 server that same server application is running on each of them. each installation of server app has its own configurations, data and settings. other client users or client applications can connect to this servers and communicate with them.

but now i need to share some data between this server applications. for example server A needs to access to information of server B's customers. or main admin of system want to see some information of server C. other scenario may be back up/sync. each server with 4th server with special application.

this servers are in different places.

this application are created by PHP and servers are ubuntu servers, each server has its own static IP.

how i can do this? any one done same thing before and how? witch is the more secure way?

ressan.ir
CI is nice