A Slim application provides built-in support for HTTP caching using the resource’s last modified date. When you specify a last modified date, Slim tells the HTTP client the date and time the current resource was last modified. The HTTP client will then send a If-Modified-Since header with each subsequent HTTP request for the given resource URI. If the last modification date you specify matches the If-Modified-Since HTTP request header, the Slim application will return a 304 Not Modified HTTP response that will prompt the HTTP client to use its cache; this also prevents the Slim application from serving the entire markup for the resource URI saving bandwidth and response time.

Setting a last modified date with Slim is very simple. You only need to invoke the Slim application’s lastModified() method in your route callback passing in a UNIX timestamp of the last modification date for the given resource. Be sure the lastModified() method’s timestamp updates along with the resource’s last modification date; otherwise, the browser client will continue serving its outdated cache.

$app->get('/foo', function () use ($app) {
    echo "This will be cached after the initial request!";