doubt it can be done. besides, if you're uploading 500 files then it would take forever, and it'd be highly unlikely that the connection would remain stong enough for this to work (let alone server script timeouts).
And i wouldn't be too surprised that it cant be done or it's difficult - the stuff that's easy is always stuff people need to do quite a bit. How often do you know of people uploading 500 files to a website?
here's your best bet - if your server can deal with zip files (think you need the zip library compiled), which some can, then you could upload all the files in one zip and then get the server to extract all the files from it. not too sure how it's done, but i have heard that php can do it. have a look at zip_open() and zip_read() - it's probably your best bet.