应开发需求安装Composer，Composer是PHP中用来管理依赖（dependency）关系的工具。可以在自己的项目中声明所依赖的外部工具库（libraries），Composer会帮你安装这些依赖的库文件。
1.获取composer
curl -sS https://getcomposer.org/installer | php
注意：如果是编译安装的php，此处需指定php文件位置，上面代码就需要变成
curl -sS https://getcomposer.org/installer | /usr/local/php/bin/php
如果不指定使用默认php程序。
2.移动到启动目录
mv composer.phar /usr/local/bin/composer
3.验证安装
composer -v
错误1：
[root@localhost ~]# composer require topthink/think-swoole
Do not run Composer as root/super user! See https://getcomposer.org/root for details
解决1：
不可以用root用户启动，切换成非root用户启动即可
错误2：
[www@ ~]$ composer require topthink/think-swoole
PHP Warning: PHP Startup: Invalid library (maybe not a PHP library) 'gettext.so' in Unknown on line 0
[ErrorException]
file_put_contents(./composer.json): failed to open stream: Permission denied
require [--dev] [--prefer-source] [--prefer-dist] [--no-progress] [--no-suggest] [--no-update] [--no-scripts] [--update-no-dev] [--update-with-dependencies] [--update-with-all-dependencies] [--ignore-platform-reqs] [--prefer-stable] [--prefer-lowest] [--sort-packages] [-o|--optimize-autoloader] [-a|--classmap-authoritative] [--apcu-autoloader] [--] [<packages>]...
解决2：
权限问题，用户需要有对当前文件夹的修改权限
请求依赖正确执行结果：
- 更换国内镜像
composer config -g repo.packagist composer https://packagist.laravel-china.org
- 执行
composer require topthink/think-swoole
注意：不更换国内镜像可能会下载失败。