public static function columnSum(array $data, array $columns)
    {
        $result = array();

        foreach ($columns as $column) {
            foreach ($data as $collection) {
                if (isset($collection[$column])) {
                    if (isset($result[$column])) {
                        $result[$column] += $collection[$column];
                    } else {
                        $result[$column] = $collection[$column];
                    }
                }
            }
        }

        return $result;
    }