注意：PHP $ _SESSION的未定义索引

Question

这是我收到错误的代码： 注意：未定义的索引：第3行的C：\ xampp \ htdocs \ Jobs \ session.php中的sbjbs_username

<?php
session_start();
$_SESSION['sbjbs_username'];
$_SESSION['sbjbs_userid']; 
$_SESSION['sbjbs_memtype'];
$_SESSION['sbjbs_emp_username'];
$_SESSION['sbjbs_emp_userid'];
//$_SESSION['sbjbs_emp_username'];
//$_SESSION['sbjbs_emp_userid'];
$_SESSION['softbiz_jbs_provided'];
//$_SESSION['ID'];
//$_SESSION['ID'];
?>