What in the world of facebook is rsrc.php!?

http://static.ak.fbcdn.net/rsrc.php/117600/css/base.css

http://static.ak.fbcdn.net/rsrc.php/z4HBM/hash/3krgnmig.swf

http://b.static.ak.fbcdn.net/rsrc.php/z23ZQ/hash/3ls2fki5.xml

http://static.ak.fbcdn.net/rsrc.php/z7O0P/hash/4hw14aet.png

What does rsrc.php really does? I know that rsrc stands for resource and rsrc.php/z[random]/hash or css/file.extenstion loads a file from somehwere.

Assuming /hash/ or /css/ is a folder which keeps the files like .xml .png .swf but whats with z[random] thing and why they want to load a file from a php? Is it for something like version control for the file or what? If so how to do it (in a simpler way)?