PHP的缓存控制设置不起作用

Question

i have the following function in my header

function header_alter($file)
{
    $timestamp=filemtime($file);
    $tsstring = gmdate('D, d M Y H:i:s ', $timestamp) . 'GMT';
    $etag = $timestamp;
    $if_modified_since = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? $_SERVER['HTTP_IF_MODIFIED_SINCE'] : false;
    $if_none_match = isset($_SERVER['HTTP_IF_NONE_MATCH']) ? $_SERVER['HTTP_IF_NONE_MATCH'] : false;
    if ((($if_none_match && $if_none_match == $etag) || (!$if_none_match)) &&
        ($if_modified_since && $if_modified_since == $tsstring))
    {
        $arr[] = 'HTTP/1.1 304 Not Modified';       
    }
    else
    {
        $arr[] = "Last-Modified: $tsstring";
        $arr[] = "ETag: \"{$etag}\"";
    }
    $arr[] = "Cache-Control: max-age=3600";
    $arr[] = 'Expires: ' . date('D, d M Y H:i:s', time() + (3600)) . ' GMT';
    return $arr;
}

but when i use the below code to see headers

foreach (getallheaders() as $name => $value) {
    echo "$name: $value\n";
}

Host: localhost
Connection: keep-alive
**Cache-Control: max-age=0**
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.63 Safari/537.36
Accept-Encoding: gzip, deflate
Accept-Language: en-GB,en-US;q=0.8,en;q=0.6
If-None-Match: "1387557104"
If-Modified-Since: Fri, 19 Dec 2013 16:31:44 GMT

The cache control is not updated. Thnx for the help guys.

Answer 1

We can use Cache-Control: max-age=… to inform browser that the component won't be changed for defined period. This way we avoid unneeded further requests if browser already has the component in its cache and therefore primed-cache page views will be performed faster. Modern browsers able to cache static files even without any cache control headers using some heuristic methods but they will do it more efficient if we define caching headers implicitly.

For Apache2 you can enable max-age using mod_expires:

ExpiresActive On
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
ExpiresByType text/css "access plus 1 month"
ExpiresByType text/javascript "access plus 1 month"
ExpiresByType application/x-javascript "access plus 1 month"
ExpiresByType application/x-shockwave-flash "access plus 1 month"

For Lighttpd there is mod_expire module.
Enable it in server.modules section:

server.modules = (
...
"mod_expire",
...
)

Then add following directives for directories with static files:

$HTTP["url"] =~ "^/images/" {
expire.url = ( "" => "access 30 days" )
}

Max-age for Nginx server can be enabled using ngx_http_headers_module: expires max;

Now web server sends the caching header for static files: Cache-Control: max-age=2592000

In case of design change we should prevent using outdated content that browsers have in their caches. This can be done by adding file versions to filenames: script.js -> script1.js -> script2.js -> ... etc

Cache-control: max-age can be useful also when we output HTML. Imagine pages generated by PHP that changed not so often, once per day or even longer. But browsers still have to download HTML every page view. We can improve it by sending max-age value in PHP.

header('Cache-Control: max-age=28800');

• Reference http://www.webscalingblog.com/performance/caching-http-headers-cache-control-max-age.html