Rewriting an anonymous function in php 7.4

Just like Barmar said, you can't use $f from the outside scope, because when the implicit binding takes place $f is still undefined.

There is nothing stopping you from passing it later as a parameter.

$f = fn($f, $n) => $n == 1 ? 1 : $n * $f($f, $n - 1);
echo $f($f, 5); // 120

The way arrow functions work, is that during definition time they will use by-value binding of the outer scope's variables.

As already mentioned, arrow functions use by-value variable binding. This is roughly equivalent to performing a use($x) for every variable $x used inside the arrow function. - https://wiki.php.net/rfc/arrow_functions_v2

The assignment of the closure to the variable $f happens after closure's definition and the variable $f is undefined prior to it.

As far as I am aware, there isn't any mechanism to bind by reference while defining arrow functions.

I don't think you can rewrite the function as an arrow function.

An arrow function will capture the value of any external variables by value at the time the function is created. But $f won't have a value until after the function is created and the variable is assigned.

The original anonymous function solves this problem by capturing a reference to the variable with use (&$f), rather than use ($f). This way, it will be able to use the updated value of the variable that results from the assignment.

I think I just found one of the legitimate (no?) uses of $GLOBALS

$f = fn ($n) =>($n == 1) ? 1 : $n * $GLOBALS['f']($n - 1);
echo $f(5); // 120

Sidenote: what if $n < 1 ?