composer install,composer update,composer require 傻傻分不清?

简单解释:
composer install - 如有 composer.lock 文件,直接安装,否则从 composer.json 安装最新扩展包和依赖;
composer update - 从 composer.json 安装最新扩展包和依赖;
composer update vendor/package - 从 composer.json 或者对应包的配置,并更新到最新;
composer require new/package - 添加安装 new/package, 可以指定版本,如: composer require new/package ~2.5.

详细说明:正确的 Composer 扩展包安装方法