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