In a recent post to his blog Ryan Gantt looks at an interesting way to get around a limitation in PHP dealing with anonymous recursion and closures that throws a Fatal error when called.
Turns out that variables called as functions must be an instance of Closure, an instance of a class which implements __invoke(), or a string representing a named function in the global namespace. In the anonymous function body above, $fibonacci is none of these. It is an undeclared, free variable in the closure created by the anonymous function. At the time when it’s called, it hasn’t been bound—hence the Notice that you would have gotten if error reporting were set at a high enough threshold - and therefore can’t be called as anything, let alone as a function.
He tried using the "use" functionality PHP closures have to bring a variable/object/etc into the scope of the running function, but it still threw an error. As it turns out, the combination of "use"-ing the object and calling it by reference handles things correctly. He takes this method and applies it in two examples - one call in an array_map function and another in an array_reduce.