FOF30\Utils\Collection::sum PHP Метод

    public function sum($callback)
    {
        if (is_string($callback)) {
            $callback = $this->valueRetriever($callback);
        }
        return $this->reduce(function ($result, $item) use($callback) {
            return $result += $callback($item);
        }, 0);
    }

 public function testGettingSumFromEmptyCollection()
 {
     $c = new Collection();
     $this->assertEquals(0, $c->sum('foo'));
 }