Actually the title says it all,
However, FMS is a file 'server', not a file 'client'. FMS-A cannot see files on FMS-B because it isn't a client and cannot go looking on the network for files it doesn't serve.
When defining the TO (Table Occurrence) and TOG (Table Occurrence Group) you are using FMP client, which can access multiple hosts at the same time and reconcile where the data come from.
Your PHP can be programed to get the data from whichever server it needs to. You just have to point each request to the correct serve for the desired data. By using ODBC designations or complete paths to either the IP or the DNS of the FMS you can get data into the PHP from multiple FMS because the PHP can act like client software.