Is my implementation of HTTP Conditional Get answers in PHP is OK?

Question

After searching a lot, reading every tutorials I've found and asking some questions here, I've finally managed to answer corrctly (at least I think) to if-none-match and if-modified-since HTTP requests. To do a quick recap, this is what I do on every pages cacheable: ``` session_cache_limiter('public'); //Cache on clients and proxies session_cache_expire(180); //3 hours header('Content-Type: ' . $documentMimeType . '; charset=' . $charset); header('ETag: "' . $eTag . '"'); //$eTag is a MD5 of $currentLanguage + $lastModified if ($isXML) header('Vary: Accept'); //$documentMimeType can be either application/xhtml+xml or text/html for XHTML (based on $_SERVER['HTTP_ACCEPT']) header('Last-Modified: ' . $lastModified); header('Content-Language: ' . $currentLanguage); ``` Also, every page have it's own URL (for every languages). For example, "index.php" will be served under URL "/en/home" in English and "/fr/accueil" in French. My big problem was to answer a "304 Not Modified" to if-none-ma…

Accepted Answer

- It's not quite correct. Please take a look at the algorithm: alt text http://img532.imageshack.us/img532/1017/cache.png

- 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)

Here is the function that might help:

```
function isModified($mtime, $etag) {
    return !( (
        isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])
        && 
        strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= $mtime
    ) || (
        isset($_SERVER['HTTP_IF_NONE_MATCH'])
        && 
        $_SERVER['HTTP_IF_NONE_MATCH'] == $etag
    ) ) ;
}
```

I suggest that you take a look at the following article: http://www.peej.co.uk/articles/http-caching.html

Update:

[AlexV] Is is even possible to receive if-none-match AND if-modified-since at the same time?

You can definitely have both set. However:

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.

RFC2616 #14.26

Example values (W stands for 'weak'; read more in RFC2616 #13.3.3):

```
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: *
```

As a special case, the value "*" matches any current entity of the resource.

Is my implementation of HTTP Conditional Get answers in PHP is OK?

Solution

Problem