On DZone.com there's a new tutorial from Giorgio Sironi about reusing closures with the help of functors (a special kind of object instancing done in PHP with the help of __invoke).

I like PHP closures and their superset, anomyous functions, as they implement the Command pattern very well when used correctly. However I feel that sometimes they are: difficult to reuse and difficult to force contracts on. [...] What if we wanted instead, a closure which we can instance even more than one time (maybe with different variables), and that we could type hint?

He shows how to make this possible with a functor created using the __invoke magic method of PHP to handle the request to an object like a function. He includes some sample code to show it in action - a basic callback (SquareCallback) and how it compares to calling a normal closure. It also shows something a bit more technical, an "AdderCallback" class that can be defined as a type hint on a function.