问如何在codeigniter中重置单个会话数组索引值？

EN

//Create or setup the array of the fields you want to update.
$newFields = array('avatar' = > 'image01.png');

//Check to see if the session is currently populated. 
if (!is_array($this->session->userdata('abc'))){
    //...and if it's not - set it to a blank array
    $this->session->set_userdata('abc',array());
}

//Retrieve the existing session data
$existing_session = $this->session->userdata('abc');

//Merge the existing data with the new data
$combined_data = array_merge($this->session->userdata('abc'), $newFields);
//update the session
$this->session->set_userdata('abc',$combined_data);

$user_data['display_name'] = "John Doe";

$user_data['avatar'] = "./images/user144.jpg";

$this->session->set_userdata('user_data',$user_data);

$user_data = $this->session->userdata('user_data');

$user_data['avatar'] = $new_avatar;

$this->session->set_userdata('user_data', $new_avatar);