php closures: why the 'static' in the anonymous function declaration when binding to static class?

found the difference: you can't bind static closures to object, only change the object scope.

class foo { }

$cl = static function() { };

Closure::bind($cl, new foo); // PHP Warning:  Cannot bind an instance to a static closure
Closure::bind($cl, null, 'foo') // you can change the closure scope

Static Closures, like any other static method, cannot access $this.

Like any other method, a non-static Closure that does not access $this will generally work in a static context.

As you've noticed, it doesn't really matter.

It's like using the static keyword on a class method. You don't necessarily need it if you don't reference $this within the method (though this does violate strict standards).

I suppose PHP can work out you mean the Closure to access A statically due to the null 2nd argument to bind()