试试这个代码:控制器

  public function changepasswordcheckagain() {

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

        $arr = array();
        $old_password = $this->input->post('old_password');
        $new_password = $this->input->post('new_password');

        $this->load->model('User_model');
        $where = ['user_id' => $user_id];

        //$data['password'] ---> column name 
        //$data['password' => $new_password] ---> new value of column
        $data = [
            'password' => $new_password
        ];

        $result = $this->User_model->checkPasswordfor($where, $data);

        echo json_encode($result);
        if ($result == TRUE) {

            echo json_encode('success');exit; 
        } else {
                  echo json_encode($result);
            echo json_encode('fail');exit; 

        }
  }

模型

   public function checkPasswordfor($arr)
   {
        $this->db->where($where);
        if($this->db->update('table name',$data))
        {
            return TRUE;
        }
        else
            return FALSE;
}