用PHP使用cURL和HTTP头正确缓存XML提要

我使用cURL获取XML提要并将其保存到XML文件中，如下所示

if (file_exists($filename) && (filemtime($filename) > time() - 30)) {
    // load cached XML from file
    $output = simplexml_load_file($filename);
    return $output;
} else {
    // Initiate the curl session
    $ch = curl_init();
    // Set the URL
    curl_setopt($ch, CURLOPT_URL, $url);
    // Removes the headers from the output
    curl_setopt($ch, CURLOPT_HEADER, 0);
    // Callback to get custom headers
    curl_setopt($ch, CURLOPT_HEADERFUNCTION, array(&$this,'readHeader'));
    // Return the output instead of displaying it directly
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    // Execute the curl session
    $output = curl_exec($ch);
    // Return headers
    $headers = curl_getinfo($ch);
    // Merge headers into one array
    $this->response_meta_info = array_merge($headers, $this->response_meta_info);
    // Close the curl session
    curl_close($ch);
    // Cache feed
    file_put_contents($filename, $output);
    // XML to object
    $output = simplexml_load_string($output);
    // Return the output as an array
    return  $output;
}

返回以下http报头

Array(
    [url] => http://example.com
    [content_type] => text/xml
    [http_code] => 200
    [header_size] => 425
    [request_size] => 108
    [filetime] => -1
    [ssl_verify_result] => 0
    [redirect_count] => 0
    [total_time] => 0.677605
    [namelookup_time] => 0.181104
    [connect_time] => 0.342955
    [pretransfer_time] => 0.343036
    [size_upload] => 0
    [size_download] => 6579
    [speed_download] => 9709
    [speed_upload] => 0
    [download_content_length] => 6579
    [upload_content_length] => 0
    [starttransfer_time] => 0.515733
    [redirect_time] => 0
    [expires] => Wed, 23 Nov 2011 15:49:24 GMT
)

我想使用[expires] => Wed, 23 Nov 2011 15:49:24 GMT标头，而不是使用这个if语句(file_exists($filename) && (filemtime($filename) > time() - 30)) {现在减去30秒的时间，因为它只会在需要时更新缓存。

但是我不知道怎么做。