PHP文件生成的图片无法使用CDN缓存的解决方法

//检测有没改变
if (isset($_SERVER['HTTP_IF_NONE_MATCH'])){
    $etag = $_SERVER['HTTP_IF_NONE_MATCH'];
    if (md5($this->image) === $etag){
        header("HTTP/1.1 304 Not Modified");
        exit;
    }
}
 
header("Last-Modified: " . gmdate("D, d M Y H:i:s", strtotime('2011-1-1'))." GMT");
//输出etag头
header('etag:' . md5($this->image));
header('Cache-Control:max-age=2592000');echo $this->image;

fastcgi_param  CACHE_ETAG    $http_if_none_match;