php 了解 If-Modified-Since HTTP 标头
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Understand If-Modified-Since HTTP Header
提问by Mike
I am looking at a Caching library that is trying to use the If-Modified-Sinceheader of a request object. The problem is this header never gets set, it is always blank which makes sense to me seeing how it is a REQUEST.
我正在查看一个尝试使用If-Modified-Since请求对象标头的缓存库。问题是这个标题永远不会被设置，它总是空白，这对我来说是有意义的，因为它是一个请求。
How can you force a request to have a If-Modified-Sinceheader? Or am I way off for what this does.
如何强制请求具有If-Modified-Since标头？或者我对它所做的事情有什么看法。
Here is the function I am referring to.
这是我所指的功能。
public function isNotModified(Request $request)
{
$lastModified = $request->headers->get('If-Modified-Since');
$notModified = false;
if ($etags = $request->getEtags()) {
$notModified = (in_array($this->getEtag(), $etags) || in_array('*', $etags)) && (!$lastModified || $this->headers->get('Last-Modified') == $lastModified);
} elseif ($lastModified) {
$notModified = $lastModified == $this->headers->get('Last-Modified');
}
if ($notModified) {
$this->setNotModified();
}
return $notModified;
}
回答by BalusC
A request with If-Modified-Sinceonly makes sense if the client already has a resource which is obtained along with a response that has a Last-Modifiedheader in combination with headers which allow browser caching like a Cache-Controland/or Pragmavalue containing public.
If-Modified-Since仅当客户端已经拥有一个资源以及响应时，请求才有意义，该响应具有Last-Modified标头与标头组合，允许浏览器缓存像 aCache-Control和/或Pragma包含public.
Also, I've noticed that some browsers does not include If-Modified-Sincewhen the original response also contained an ETagheader. The browser will instead use If-None-Matchto test it.
另外，我注意到某些浏览器不包括If-Modified-Since原始响应何时还包含ETag标头。浏览器将使用If-None-Match它来测试它。
See also:
也可以看看：
回答by alienhard
First you have to make sure the initial response is cached in the first place (I answeredthis in another, related question.
首先，您必须确保首先缓存初始响应（我在另一个相关问题中回答了这个问题。
Try to set the following fields:
尝试设置以下字段：
Last-Modified: Wed, 16 Feb 2011 13:52:26 GMT
Expires: -1
Cache-Control: must-revalidate, private
Last-Modifiedis needed as a validator (do not sendETagif you want to test forIf-Modified-Since)Expires -1tells that the resource is stale and must always be revalidatedCache-Controlmust not include no-cache nor no-store
Last-Modified需要作为验证器（ETag如果要测试，请不要发送If-Modified-Since）Expires -1告诉资源是陈旧的，必须始终重新验证Cache-Control不得包含 no-cache 或 no-store
When you send these headers on the initial HTTP/200response, on subsequent requests, the browser should send conditional requests that include the If-Modified-Sinceheader.
当您在初始HTTP/200响应中发送这些标头时，在后续请求中，浏览器应发送包含If-Modified-Since标头的条件请求。
