使用国内镜像：https://docs.phpcomposer.com/
composer config -g repo.packagist composer https://packagist.phpcomposer.com
Linux下出现错误：
- topthink/framework v6.0.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
安装：
sudo apt-get install php7.4-mbstring
然后安装
composer create-project topthink/think tp