I'm not sure how to do this with strictly PHP (I am doubtful it is possible).
But, a general solution... Mount the file server location you want to use (say /root/myfiles/ on the file server), to place on the web server (say /root/htdocs/myfiles/ on the web server). Doing this, any "regular" access to that spot on the web server will automatically retrieve the files off the file server.
I know this approach is possible because system administrators at my work often employ this strategy. Our satellite beams gigabytes of data to our center every day and is stored on a series of remotely connected file servers. For some web tools we use that I have developed (that require those files), our system admins simply mounted the location of the files (from the file server) to a spot on the internal web server, thus giving it access. Then, any link/PHP action/etc to this web server location will yield the file server contents.
If this was a location on the same server (outside the web root), I believe you could use Apache to "map" the location. However, given the stuff you want is on another server, I believe this goes beyond just Apache/PHP and would require the mounting via the OS. Hopefully this guides you in the right direction.
EDIT: Re-reading your question, it seems like your "user" criteria complicates the matter considerably. I am guessing depending on the user working, the mounted file location points to a different spot on the file server; but the user working always accesses the mounted file share? Kind of like the user accessing '/myfiles/' (on user computer) is routed to '/root/userABC/files/' (on file server) [for example]? If this is about the case... you would still need to follow my mounting advice to make the files available to Apache/PHP [mount location on file server, to a location on the web server so PHP/etc has access to it]. The problem is, if the file location is dependent on the user, this complicates the matter, because you can't just mount the file location without knowing the user. Am I ~about understanding your situation correctly?