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    <ul> <li>It's not quite correct. Please take a look at the algorithm: <a href="http://img532.imageshack.us/img532/1017/cache.png" rel="noreferrer">alt text http://img532.imageshack.us/img532/1017/cache.png</a></li> <li>The solution is proxy-friendly, you may use Cache-control: proxy-revalidate to force caches to obey any freshness information you give them about a resource (only applies to shared|proxy caches)</li> </ul> <p>Here is the function that might help:</p> <pre><code>function isModified($mtime, $etag) { return !( ( isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) &amp;&amp; strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) &gt;= $mtime ) || ( isset($_SERVER['HTTP_IF_NONE_MATCH']) &amp;&amp; $_SERVER['HTTP_IF_NONE_MATCH'] == $etag ) ) ; } </code></pre> <p>I suggest that you take a look at the following article: <a href="http://www.peej.co.uk/articles/http-caching.html" rel="noreferrer">http://www.peej.co.uk/articles/http-caching.html</a></p> <p><strong>Update:</strong></p> <blockquote> <p><strong>[AlexV]</strong> Is is even possible to receive if-none-match AND if-modified-since at the same time?</p> </blockquote> <p>You can definitely have both set. However: </p> <blockquote> <p>If none of the entity tags match, then the server MAY perform the requested method as if the If-None-Match header field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request. That is, if no entity tags match, then the server MUST NOT return a 304 (Not Modified) response.</p> <p><a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26" rel="noreferrer">RFC2616 #14.26</a></p> </blockquote> <p>Example values (W stands for 'weak'; read more in <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.3.3" rel="noreferrer">RFC2616 #13.3.3</a>):</p> <pre><code>If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT If-None-Match: * </code></pre> <p>As a special case, the value "*" matches any current entity of the resource.</p>
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    1. POIs my implementation of HTTP Conditional Get answers in PHP is OK?
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    1. CONote also what RFC2616 (HTTP1.1) spec section 14.26 says about the syntax of If-None-Match. Neither this nor the original question allowed for multiple etag values or "*". That being said, this assumption is probably right 99.9% of the time.
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    2. CO@Mike Bell: Yeah I know that ETag can contain many values or * but I don't know any browser using this yet... I wonder if any system actually use this. BTW how to parse ETag when more than 1 value in it? What does * mean in ETag?
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    3. CO@St.Woland: I read the article provided and looked at your graph (nice work) and it seems to correspond to my "else" part in my 2nd snippet (am I right?). What I find odd with this is when BOTH if-none-match and if-modified-since are received only 1 of the 2 must match to reply with a 304... Is is even possible to receive if-none-match AND if-modified-since at the same time?
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