The HTTP response returned to the HTTP client will have a header. The HTTP header is a list of keys and values that provide metadata about the HTTP response. You can use the Slim application’s response object to set the HTTP response’s header. The response object has a public property headers that is an instance of \Slim\Helper\Set; this provides a simple, standardized interface to manipulate the HTTP response headers.

<?php
$app = new \Slim\Slim();
$app->response->headers->set('Content-Type', 'application/json');

You may also fetch headers from the response object's headers property, too:

<?php
$contentType = $app->response->headers->get('Content-Type');

If a header with the given name does not exist, null is returned. You may specify header names with upper, lower, or mixed case with dashes or underscores. Use the naming convention with which you are most comfortable.