Found this bug report, and the last note added by "dr_no at moonraker dot com" was rather insightful:
You can fopen() UNC paths, e.g. "\myserver\myshare\file.ext", but if you're running Apache as an NT service, it's probably running under the LocalSystem account. If you read the bug report, you'll see that this account is severely hindered when it comes to accessing network shares. So, you'll probably end up elevating the privilieges of the service (as described in the report) to accomplish this.