How to do a cacheable redirection?

Question

When users enter my website example.com, their "preferred" language is detected and they are redirected (using a 301 Moved Permanently redirection) to example.com/en/ (for English), example.com/it/ (for Italian), etc.

It works perfectly, but when I analyzed my website with the Google Page Speed tool it gave me the following advice.

Many pages, especially mobile pages, redirect users to a different URL, for instance from www.example.com to m.example.com. Making this redirect cache-able by the user's browser can speed up page load times for repeat visitors to a site.

And later it says

We recommend using a 302 redirect with a cache lifetime of one day. The redirect should include a Vary: User-Agent header as well as a Cache-Control: private header.

So my questions are, how can I do a "cache-able" redirection in PHP? Would the following be enough?

header("HTTP/1.0 302 Moved Temporarily");
header("Location: example.com/whatever");
exit;

Answer 1

This should do the trick:

header("HTTP/1.0 302 Moved Temporarily");
header("Location: example.com/whatever");
header("Cache-Control: private");
header("Vary: User-Agent, Accept-Encoding");
exit;

The recommendation for the Vary header is from this google developer page about optimizing caches (and problems with some IE < 9). Background on caching negotiated responses from RFC2616 (Header Field Definitions) where you will also find background on the Cache-control-private. A further discussion about the Vary: Header also amongst others on this stackoverflow page.

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