required/included files in general need to be on the same server as the page using the file.
I did once try to include a remote file but the remote server parsed the php before returning it and so it was no good.
In theory if the file does not end with a suffix which is linked to PHP then it could work but the file would have to be under the remote document root and would be accessible to everyone and so pose a security risk.
If you really need to share code with a remote server then it mamy be worth considering putting it on a database instead, but in general I find it easier just to have the include files locally even if it means duplicating them in several places.
