Php codeigniter中的某些错误，是真的吗？

在windows、php5.5和ci 3.1.8中进行测试 我创建了一个test.php，如下所示：

<?php
    session_start();
    $_SESSION['admin']="1";
    $str =array("_SESSION"=>array("isadmin"=>"admin","user"=>"user1"));
    extract($str);
    print_r($_SESSION);

---

提取：将变量从数组导入当前符号表中

仅当前符号表

请参见
如何使用codeigniter会话和设置最好使用CI的会话库。
<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Welcome extends CI_Controller {

/**
 * Index Page for this controller.
 *
 * Maps to the following URL
 *         http://example.com/index.php/welcome
 *    - or -
 *         http://example.com/index.php/welcome/index
 *    - or -
 * Since this controller is set as the default controller in
 * config/routes.php, it's displayed at http://example.com/
 *
 * So any other public methods not prefixed with an underscore will
 * map to /index.php/welcome/<method_name>
 * @see https://codeigniter.com/user_guide/general/urls.html
 */
public function index()
{
    $this->load->view('welcome_message');
}
public function test()
{
    session_start();
    $_SESSION['admin']="1";
    $str =array("_SESSION"=>array("isadmin"=>"admin","user"=>"user1"));
    extract($str);
    print_r($_SESSION);
}

}