You can enable debugging during application instantiation with this setting:

<?php
$app = new \Slim\Slim(array(
    'debug' => true
));

You may also enable debugging during runtime with the Slim application’s config() instance method:

<?php
$app = new \Slim\Slim();

//Enable debugging (on by default)
$app->config('debug', true);

//Disable debugging
$app->config('debug', false);

If debugging is enabled and an exception or error occurs, a diagnostic screen will appear with the error description, the affected file, the file line number, and a stack trace. If debugging is disabled, the custom Error handler will be invoked instead.