think\Response::getContent PHP Method

    public function getContent()
    {
        if (null == $this->content) {
            $content = $this->output($this->data);
            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->content;
    }