未捕获错误：在null

Question

我想通过调用$connection函数关闭close()。

class Db {
protected static $connection;

// Connect to the database
public function connect() {

  if(!isset(self::$connection)) {
    $config = parse_ini_file('database.ini');
    self::$connection = new mysqli($config['hostname'], $config['username'], $config['password'], $config['dbname']);

    if(self::$connection == false){
      //Handle error or send to the admin
      return false;
    }
    return self::$connection;
  }
}
public function close(){
  $connection = $this -> connect();
  $connection -> close();
}
}

但我得到的错误是：未捕获的错误：Call to a member function close() on null。
是因为static $connection吗？
我可以使用unset($obj)销毁对象而不必担心$connection吗？
关闭连接的正确方法是什么？