php静态文件返回304技巧分享

private function _addEtag($file) {
    $last_modified_time = filemtime($file);
    $etag = md5_file($file);
    // always send headers
    header("Last-Modified: ".gmdate("D, d M Y H:i:s", $last_modified_time)." GMT");
    header("Etag: $etag");
    // exit if not modified
    if (@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) == $last_modified_time ||
    @trim($_SERVER['HTTP_IF_NONE_MATCH']) == $etag) {
        header("HTTP/1.1 304 Not Modified");
        exit;
    }
}