_btn.addEventListener(MouseEvent.MOUSE_UP, myMouseUp);
function myMouseUp(e:Event){
   var req:URLRequest = new URLRequest("test_request.php");
   req.data = byte_array_data;
   req.method = URLRequestMethod.POST;
   req.contentType = 'application/octet-stream';
   flash.net.navigateToURL(req, "_blank");
}

<?php
$   data = $   GLOBALS['HTTP_RAW_POST_DATA'];
if (is_null($   data)) {
   echo "No data was sent";
} else {
   $   file = fopen("test.jpg", "w") or die("Can't open file");
   if(!fwrite($   file, $   data)){
      echo "Error writing to file";
   }
   print "OK\n";
   fclose($   file);
} 
?>