Having issues with realpath() and mounted file systems.

Hi. The setup is an Ubuntu Mate with a Windows IE8 VM. The Windows VM currently shares a folder for a Windows only app that has to reside in a specific location. We want to be able to mount that folder on our host. Once mounted, 'touch /home/glenn/mount/hello.txt' operates fine. (/home/glenn/mount is the rw mount for the windows share). And we can see the hello.txt file in the folder within the VM. But ... php -r "var_dump(realpath('/home/glenn/mount'));" returns false. unmounting the drive and rerunning the php command returns true. The folder has rwxr_xr_x permissions (0755). I think everything is OK, but I don't understand why realpath() and associated functions (file_exists(), etc.) all return false. Any suggestions/reading/etc. would be greatly appreciated. Regards, Richard.