you might also want to look at:
http://rapidget.linux.spb.ru/,
http://www.rapidleech.com/ and
http://www.syi.cc/viewtopic.php?pid=458383
these scripts have been modified to deal with the changes rapidshare.com have made, switching to a cookie authentication method. The basic function is to request a url with the header Cookie: user=$rs_user-$rs_pass, and a download url is returned on the Location: header. So the computer which requested the download with the cookie can only download from the server.
Two options i see for your script is to make it some how stream the content, as in the server grabs the data then send to the member.
Or when the member requests the URL the script starts a download and saves it to a location and returns the link to the member when the download is complete.
Not an expert with PHP, but some ideas to get you started

hope you can release a new version soon. Thanks for your effort