private function readEBMLelementData($length, $check_buffer=false) {
    if ($check_buffer && !$this->getid3_matroska::EnsureBufferHasEnoughData($length)) {
      return false;
    }
    $data = substr($this->EBMLbuffer, $this->current_offset - $this->EBMLbuffer_offset, $length);
    $this->current_offset += $length;
    return $data;
  }