function data_encode($data, $keyprefix = "", $keypostfix = "") {
  assert( is_array($data) );
  $vars=null;
  foreach($data as $key=>$value) {
    if(is_array($value)) $vars .= data_encode($value, $keyprefix.$key.$keypostfix.urlencode("["), urlencode("]"));
    else $vars .= $keyprefix.$key.$keypostfix."=".urlencode($value)."&";
  }
  return $vars;
}