A Slim application provides a set of hooks to which you can register your own callbacks to it.
A “hook” is a moment in the Slim application lifecycle at which a priority list of callables assigned to the hook will be invoked. A hook is identified by a string name.
A “callable” is anything that returns true for is_callable(). A callable is assigned to a hook and is invoked
when the hook is called. If multiple callables are assigned to a single hook, each callable is invoked in the order
assigned.