isset（$ _ SESSION ['id']）工作不正常[关闭]

I am trying to use the condition:

isset($_SESSION['id'])

So i just tried

echo isset($_SESSION['id']); and echo $_SESSION['id'];

But this gives output different. In fact true result gives by

echo $_SESSION['id'];

Any one have idea why this is happening?
Note: I use session_start(); at the beginning of page.

isset is used in if condition, in case you want to check if session is set or not:

<?php

    if(isset($_SESSION['id'])) { // check if session named 'id' is exist
       echo $_SESSION['id'];
    }

?>