> essentially Closure::fromCallable() and $RF->getClosure() should always return the same Closure object
I disagree. Both of them are documented to return "new" (Closure::fromCallable) or "dynamically created" (ReflectionFunction::getClosure) closures - copies of the original, not shallow references to it. And that implies copies of any static variables it may have, which is why the first fromCallable shows [0,1] instead of [1].

Regular functions and closures are two separate things: a function is not a closure, and a closure is not a function*. Creating a closure from a function is not like creating an alias or performing some sort of import (cf. traits in classes).

How about we clarify the behavior in the documentation?

* It's an object (\Closure) that behaves like a function, and is defined using syntax that resembles that of a function, but is not actually a function.

yes you can document the incomplete functionality if you want, but it won't make the user experience any better

comparing objects seems to me a very basic feature

I don't really care about statics as I rarely use them, but again I doubt the current behavior is what people expect

what's the point of Closure::fromCallable("foo") then? if I did function(...$a){ return foo(...$a); } I'd at least have the static variables working

Static variables should be shared. It should not make a difference whether a callable is invoked directly or indirected through Closure::fromCallable(). The current behavior will also negatively interact with future plans for more first-class callable references based on closure objects.

As for returning the same object, I don't think we're going to add such a guarantee. In any case, it should be part of a separate request, as it is unrelated to the static variable bug (which are due to internal state management, not which object is being used.)

> Static variables should be shared.

I agree.  But what about the current working of function statics
in subclasses, e.g. <https://3v4l.org/4rICv>?

@cmb: That case is something of a mess, which depends on the order in which you do things. Bob had a patch that normalized this (https://github.com/php/php-src/pull/1899), though it did not go anywhere...