private function _populate_cache()
    {
        if ($this->_use_json) {
            if (is_readable($this->_cache_file) &&
            ($loaded = @json_decode(Common::getContents($this->_cache_file, ''), true)) && is_array($loaded)) {
                $this->_existing_cache_vals = $loaded;
            }
        } else {
            if (is_readable($this->_cache_file) &&
            ($loaded = @unserialize(Common::getContents($this->_cache_file, false))) && is_array($loaded)) {
                $this->_existing_cache_vals = $loaded;
            }
        }
    }