Amp\Artax\FormBody::sumMultipartFieldLengths PHP Method

    private function sumMultipartFieldLengths(array $fields)
    {
        $lengths = [];
        foreach ($fields as $field) {
            if (is_string($field)) {
                $lengths[] = strlen($field);
            } else {
                $lengths[] = $field->getLength();
            }
        }
        $promisor = new Deferred();
        \Amp\all($lengths)->when(function ($error, $result) use($promisor) {
            if ($error) {
                $promisor->fail($error);
            } else {
                $promisor->succeed(array_sum($result));
            }
        });
        return $promisor->promise();
    }