Description:
------------
Using self::$invokables[$method](...$arguments); where self::$invokables is an array of objects that implement __invoke() does not work. PHP wrongly complains that self::$invokables is not declared.

Test script:
---------------
<?php
class c
{
    private static $invokables = [];

    public static function registerInvokable($name, $invokable)
    {
        self::$invokables[$name] = $invokable;
    }

    public function __call($method, array $arguments)
    {
        // works
        $method = self::$invokables[$method];
        $method(...$arguments);

        // does not work        
        self::$invokables[$method](...$arguments);
    }
}

class i
{
    public function __invoke(...$arguments)
    {
        print_r($arguments);
    }
}

c::registerInvokable('foo', new i);

$c = new c;
$c->foo(1, 2);


Expected result:
----------------
Array
(
    [0] => 1
    [1] => 2
)
Array
(
    [0] => 1
    [1] => 2
)


Actual result:
--------------
Array
(
    [0] => 1
    [1] => 2
)

Notice: Undefined variable: invokables in /home/sb/test.php on line 18

Fatal error: Function name must be a string in /home/sb/test.php on line 18