Seems simple from what you asked, so maybe I'm missing something,, but:
Just get the script to be included from the PATH_INFO
$includefilename = substr($PATH_INFO,1);
$includefilename .= '.dat';
include("$includefilename"); #( you can figure out the directory if needed)
Is "someserver.net" in your example your server? If yes, the above should work. If not, then look up & use fopen() instead of include().
Cheers,
Ben