think\Response::content PHP Method

    public function content($content)
    {
        if (null !== $content && !is_string($content) && !is_numeric($content) && !is_callable([$content, '__toString'])) {
            throw new \InvalidArgumentException(sprintf('variable type error： %s', gettype($content)));
        }
        $this->content = (string) $content;
        return $this;
    }