php-mongo帮助文档
https://docs.mongodb.com/php-library/master/reference/method/MongoDBCollection-aggregate/
http://php.net/manual/zh/mongocollection.find.php
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https://docs.mongodb.com/php-library/master/reference/method/MongoDBCollection-aggregate/
http://php.net/manual/zh/mongocollection.find.php