array('B', 'parent::who') is not valid: the second element must be the name of a method. array('B', 'who') is the correct form.

According to
http://us1.php.net/manual/en/language.types.callable.php

It is valid:

// Type 5: Relative static class method call (As of PHP 5.3.0)
class A {
    public static function who() {
        echo "A\n";
    }
}

class B extends A {
    public static function who() {
        echo "B\n";
    }
}

call_user_func(array('B', 'parent::who')); // A

Interesting. I had no idea that feature existed. Just checked with 5.5.6 and it has the same behavior.

Looks like a bug then as it's not showing any class at all. However I'm not sure which class it should show: could be A if it resolved as B::parent's who, or it could be B if it resolved as B and then parent::who.

I was thinking it should output B, but yea, you are right, it simply means B's parent who, but doesn't mention the calling scope. The documentation should show more details on parent::who, what exactly it means.

here is a new little test snippet:

class A
{
    public static function who()
    {
        $obj = new static; 
        var_dump($obj);
    }
}
class B extends A
{
    public static function who()
    {
        parent::who();
    }
}
B::who();          //output object(B)[1]
call_user_func(array('B', 'parent::who'));     //output Fatal error: Cannot access static:: when no class scope is active

This means get_called_class() does it job correctly, as it is out of scope and returns null.

However, the doc should gives more details on "parent::who"

The example calls this feature "relative static class method
call", and as such it should behave identically to calling
A::who() directly, but it doesn't: <https://3v4l.org/vbuVd>.
Interestingly, calling the callable $method directly fails
completely.

This even causes an assertion failure nowadays:

php: /home/nikic/php/php-src/Zend/zend_vm_execute.h:35335: ZEND_GET_CALLED_CLASS_SPEC_UNUSED_UNUSED_HANDLER: Assertion `!((execute_data)->func)->common.scope' failed.