CI_Output::_write_cache PHP Method

    public function _write_cache($output)
    {
        $CI =& get_instance();
        $path = $CI->config->item('cache_path');
        $cache_path = $path === '' ? APPPATH . 'cache/' : $path;
        if (!is_dir($cache_path) or !is_really_writable($cache_path)) {
            log_message('error', 'Unable to write cache file: ' . $cache_path);
            return;
        }
        $uri = $CI->config->item('base_url') . $CI->config->item('index_page') . $CI->uri->uri_string();
        if (($cache_query_string = $CI->config->item('cache_query_string')) && !empty($_SERVER['QUERY_STRING'])) {
            if (is_array($cache_query_string)) {
                $uri .= '?' . http_build_query(array_intersect_key($_GET, array_flip($cache_query_string)));
            } else {
                $uri .= '?' . $_SERVER['QUERY_STRING'];
            }
        }
        $cache_path .= md5($uri);
        if (!($fp = @fopen($cache_path, 'w+b'))) {
            log_message('error', 'Unable to write cache file: ' . $cache_path);
            return;
        }
        if (flock($fp, LOCK_EX)) {
            // If output compression is enabled, compress the cache
            // itself, so that we don't have to do that each time
            // we're serving it
            if ($this->_compress_output === TRUE) {
                $output = gzencode($output);
                if ($this->get_header('content-type') === NULL) {
                    $this->set_content_type($this->mime_type);
                }
            }
            $expire = time() + $this->cache_expiration * 60;
            // Put together our serialized info.
            $cache_info = serialize(array('expire' => $expire, 'headers' => $this->headers));
            $output = $cache_info . 'ENDCI--->' . $output;
            for ($written = 0, $length = strlen($output); $written < $length; $written += $result) {
                if (($result = fwrite($fp, substr($output, $written))) === FALSE) {
                    break;
                }
            }
            flock($fp, LOCK_UN);
        } else {
            log_message('error', 'Unable to secure a file lock for file at: ' . $cache_path);
            return;
        }
        fclose($fp);
        if (is_int($result)) {
            chmod($cache_path, 0640);
            log_message('debug', 'Cache file written: ' . $cache_path);
            // Send HTTP cache-control headers to browser to match file cache settings.
            $this->set_cache_header($_SERVER['REQUEST_TIME'], $expire);
        } else {
            @unlink($cache_path);
            log_message('error', 'Unable to write the complete cache content at: ' . $cache_path);
        }
    }

 function _write_cache($output)
 {
     if ($this->is_logged_in) {
         // don't write cache if user is logged in
         return false;
     } else {
         // user isn't logged in, proceed as normal
         parent::_write_cache($output);
     }
 }