Silex DI

Application.php

    /**
     * The Application(which is an instance of Pimple) sets up it's required dependencies, one of which is a logger
     */
    $this['logger'] = null;

    /** snip */
    
    /**
     * The controller resolver takes an optional logger
     */
    $this['resolver'] = $this->share(function () use ($app) {
        return new ControllerResolver($app, $app['logger']);
    });

MonologServiceProvider.php

            /**
             * Providers get passed the Application instance to register their services (and their dependencies)
             * 
             * The Monolog provider adds it's own services to the container, and then overrides default logger
             */
            $app['logger'] = function () use ($app) {
                return $app['monolog'];
            };