protected function createWorkingDirectory()
  {
    $this->working_directory = $this->working_base_directory."phpcrawl_tmp_".$this->crawler_uniqid.DIRECTORY_SEPARATOR;
    
    // Check if writable
    if (!is_writeable($this->working_base_directory))
    {
      throw new Exception("Error creating working directory '".$this->working_directory."'");
    }
    
    // Create dir
    if (!file_exists($this->working_directory))
    {
      mkdir($this->working_directory);
    }
  }