304 Not Modified

Redirection: the client has to take another step, usually following a Location header, to complete the request.

When a server should send it. In answer to a conditional GET or HEAD (If-None-Match or If-Modified-Since) when the client's cached copy is still current. Send the same validators (ETag, Last-Modified) and caching headers a 200 would have had, and no body.

Common causes. The browser asked with If-None-Match or If-Modified-Since and nothing changed.

What to do. Nothing; this is caching working. If users see stale content, the server is sending the same ETag or date after changes.

Browsers and HTTP clients. Browsers reuse the copy in their cache and show it as if it had just been downloaded. Developer tools list the request with status 304; the page itself never sees the difference.

Google Search. Tells Google the content is the same as at the last crawl; the indexing pipeline may recalculate signals for the URL, but otherwise it has no effect on indexing. Source: Google Search Central, How HTTP status codes, and network and DNS errors affect Google Search.

Caching. Not stored on its own: a cache uses it to refresh the headers of the copy it already holds and keeps serving that copy (RFC 9111 section 4.3.4).

HTTP/1.1 304 Not Modified
ETag: "5f8e-6a1b2c"
Cache-Control: max-age=600

(no body)

# Static files: nginx sends 304 itself when the ETag or
# Last-Modified date matches. Keep these on (the default):
etag on;
if_modified_since exact;

$etag = '"' . md5($body) . '"';
header('ETag: ' . $etag);
if (($_SERVER['HTTP_IF_NONE_MATCH'] ?? '') === $etag) {
    http_response_code(304);
    exit;
}
echo $body;