This shows you how to enable logging so you can write stuff like $app->log->debug('this will show up in the error_log');.
<?php
--- snip %< ---
$app = new \Slim\Slim(array(
'log.enabled' => true,
'log.level' => \Slim\Log::DEBUG
));
$app->log->debug('this will show up in your error-log');
--- >% /snip ---
[opt-in]