public function sign($data) {
    /* Remove signature from server. */
    unset($data['h']);

    /* Sort keys alphabetically. */
    ksort($data);

    /* Build query string to sign. */
    $n = count($data);
    $query = '';
    $i = 0;
    while(list($key, $val) = each($data)) {
      $i++;
      $query .= $key.'='.$val;
      if($i < $n) {
        $query.= '&';
      }
    }

    /* Sign. */
    $sign = hash_hmac('sha1', utf8_encode($query), base64_decode($this->clientSecret), true);
    return base64_encode($sign);
  }