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.