The HTTP response returned to the client will have a body. The HTTP body is the actual content of the HTTP response delivered to the client. You can use the Slim application’s response object to set the HTTP response’s body:

$app = new \Slim\Slim();

// Overwrite response body

// Append response body

When you overwrite or append the response object’s body, the response object will automatically set the Content-Length header based on the bytesize of the new response body.

You can fetch the response object’s body like this:

$body = $app->response->getBody();

Usually, you will never need to manually set the response body with the setBody() or write() methods; instead, the Slim app will do this for you. Whenever you echo() content inside a route's callback function, the echo()’d content is captured in an output buffer and appended to the response body before the HTTP response is returned to the client.