A Slim application provides built-in support for HTTP caching with its etag(), lastModified(), and expires()
helper methods. It is best to use one of etag() or lastModified() - in conjunction with expires() - per route;
never use both etag() and lastModified() together in the same route callback.
The etag() and lastModified() methods should be invoked in a route callback before other code; this allows Slim
to check conditional GET requests before processing the route callback’s remaining code.
Both etag() and lastModified() instruct the HTTP client to store the resource response in a client-side cache.
The expires() method indicates to the HTTP client when the client-side cache should be considered stale.