use Silex\Provider\MonologServiceProvider;
$app->register(new Silex\Provider\MonologServiceProvider(), array(
  'monolog.logfile' => __DIR__.'/log/development.log',
));

// ログ書き込み
$app['monolog']->addDebug('debug log');
$app['monolog']->addInfo('info log');
$app['monolog']->addWarning('warning log');
$app['monolog']->addError('error log');