mac 安装 composer
1.. php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
2. sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer
3. php -r "unlink('composer-setup.php');"
composer self-update --1 通过 -- 升级或降级版本
composer install --ignore-platform-reqs 忽略低版本扩展