FWIW, and not much help I imagine, you might try //machine_name/c/... etc
The manual's kinda confusing on this one, eh?
On windows, use //computername/share/filename or \computername\share\filename to check files on network shares.
And then...
Note: This function will not work on remote files as the file to be examined must be accessible via the servers filesystem.
However, I'd think that a "network file system" should be equivalent to "server's file system", so I dunno.
Have you tried calling the file with fopen(), just to see if you got an error or it returned true?