php如何实现的mongoDB单例模式操作类

class Mongo_db
{
private static $cli; 
/** 
* 不允许初始化 
*/ 
private function __construct() 
{ 
$config = Config::get('config.mongo_config'); 
if(empty($config)){ 
$this->throwError('无法连接数据库!'); 
} 
if (!empty($config["user_name"])) { 
$this->mongo = new MongoClient("mongodb://{$config['user_name']}:{$config['password']}@{$config['host']}:{$config['port']}");
}else { 
$this->mongo = new MongoClient($config['host'] . ':' . $config['port']); 
} 
} 
/** 
* 单例模式
* @return Mongo|null 
*/
public static function cli(){ 
if(!(self::$cli instanceof self)){ 
self::$cli = new self(); 
}
return self::$cli->mongo; 
} 
}
$mongo = Mongo_db::cli()->test->mycollection; // test 是选择的数据库 ， mycollection