How specific do you want? You could execute a "uname -s" command to see if it's a Unix system (and which one), or a "ver" command to see if it's Windows. These two commands would give you more specific information - which release/version number of the operating system.
More generally, you could see if it's Windows if the second character returned from PHP's constant "FILE" is a colon (Windows has drive letters "C:\" but Unix doesn't).
