A Slim application provides built-in support for HTTP caching using the resource’s last modified date. When youspecify 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 givenresource 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 useits cache; this also prevents the Slim application from serving the entire markup for the resource URI savingbandwidth 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.

<?php
$app->get('/foo', function () use ($app) {
    $app->lastModified(1286139652);
    echo "This will be cached after the initial request!";
});