public static function get($key = null)
    {
        if (null === self::$instance) {
            self::$instance = new self();
        }

        return $key ? self::$instance[$key] : self::$instance;
    }
public static function get($key = null)
    {
        if (null === self::$instance) {
            self::$instance = new self();
        }

        return $key ? self::$instance[$key] : self::$instance;
    }
public static function get ( $ key = null ) { if ( null === self :: $ instance ) { self :: $ instance = new self ( ) ; } return $ key ? self :: $ instance [ $ key ] : self :: $ instance ; }
Returns the application singleton.

@param string $key A service or parameter key.

@return Go The application singleton.
Returns the application singleton .