mongodb连接没释放的问题

按照php的话 会自动释放连接 但是就是有好多 即使php运行完毕也没被释放 为什么？
php官方http://www.php.net/manual/en/mongo.close.php
中介绍 public bool Mongo::close ( void )
This method does not need to be called, except in unusual circumstances. The driver will cleanly close the database connection when this Mongo instance goes out of scope.

If objects do not go out of scope between requests, you may wish to call this method at the end of your program to keep old connections from hanging around. However, it is probably more efficient use a persistent connection, which will automatically create a connection if needed and use it for as many requests as possible.

If you are connected to a replica set, close() will only close the connection to the primary.
不需要调用关闭。那要如何做，求指导。