如何使用Codeigniter回显单个会话

我想阅读一个会话。

这就是我设置所有会话的方式：

$data = array(
    'connection'    => true,
    'sender_db'     => $this->input->post('sender_db'),
    'sender_host'   => $this->input->post('sender_host'),
    'sender_user'   => $this->input->post('sender_user'),
    'sender_pw'     => $this->input->post('sender_pw'),
    'receiver_db'   => $this->input->post('receiver_db'),
    'receiver_host' => $this->input->post('receiver_host'),
    'receiver_user' => $this->input->post('receiver_user'),
    'receiver_pw'   => $this->input->post('receiver_pw'),
);
foreach ($data as $key => $value) {
    $this->session->set_userdata($key, $value);
}

这就是我如何阅读会话sender_host:

var_dump($this->session->get_userdata('sender_host'));

好吧，这给了我一个数组，其中所有其他会话也存储在：

array(9) {
    ["connection"]=> bool(true)
    ["sender_db"]=> string(12) "datamigrator"
    ["sender_host"]=> string(9) "localhost"
    ["sender_user"]=> string(4) "root"
    ["receiver_db"]=> string(8) "anything"
    ["sender_pw"]=> string(0) ""
    ["receiver_host"]=> string(8) "anything"
    ["receiver_user"]=> string(8) "anything"
    ["receiver_pw"]=> string(8) "anything"
}

如何只进行一次会话？

我认为问题在于我如何设置会话？！