在项目根目录创建composer.json文件，内容如下：
{
"require": {
"phpmailer/phpmailer": "~5.2",
"overtrue/wechat": "~3.1"
}
}
composer config -g repo.packagist composer https://mirrors.aliyun.com/composer/
在项目根目录下面执行 composer install
composer update
composer self-update