Response: Response

string will be send to client with content-type text/html

$app->post('/test', function() {
    return "<pre>".var_export($this->body);
});

array will be encoded and send to client with content-type application/json

$app->set('pretty');  // pretty print

$app->post('/test', function() {
    return $this->body;
});

int will be send as http status

$app->post('/test', function() {
    return 201;
});

and a little hack

$app->post('/resource', function() {
    return [201, 'resource created', 'X-HEADER' => 'foo'];
});