Questions about Isset and empty?

Recently looking at the session of the question, you need to determine whether the user login, I found the following 2 ways to successfully determine whether to log on the issue.
The code is as follows:

第一种：if(empty($_SESSION)){    echo '您还未登录，请登录
';}else{ echo '欢迎'.$_SESSION['uname'].' 这里是主页 '; echo '退出
';}第二种：if(!isset($_SESSION['uname'])){ echo '您还未登录，请登录
';}else{ echo '欢迎'.$_SESSION['uname'].' 这里是主页 '; echo '退出
';}

Question: In the second Judgment statement, if you write only $_session instead of $_session[' uname ', you will get an error, and if you use the first method, either $_session[' uname ' in the parentheses behind the empty or $_ The session is no problem, what is the reason? Which of the two methods would be better? Or is it a little safer? Thank you

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