Question

What does isset($_SESSION) mean? I found the following code snippet-

if (!isset($_SESSION)) {

// php code

}

EDIT:

I found the following snippet in define.php of the facebook style chatting script
freichat :

if (!isset($_SESSION)) {
            $this->frm_id = $_SESSION[$this->uid . 'usr_ses_id'];
            $this->frm_name = $_SESSION[$this->uid . 'usr_name'];
        }

if $_SESSION was not set previously then, how can the two session variables be assigned to other variables?

Answer 1

Summary from my comments:

• isset determines if a variable is set and is not NULL.
• Putting an ! negates the condition, so if (!false) will evaluate to true.
• $_SESSION is a superglobal and will be available on the following conditions:

Please note when working with sessions that a record of a session is not created until a variable has been registered using the session_register() function or by adding a new key to the $_SESSION superglobal array. This holds true regardless of if a session has been started using the session_start() function.

Your if (!isset($_SESSION)) will evaluate to true when no Session exists yet. It will then execute the code in the if block. However, that code will not work. If you run this snippet

<?php
var_dump($_SESSION);                // undefined notice + NULL
var_dump(isset($_SESSION));         // FALSE
if (!isset($_SESSION)) {            // !FALSE is TRUE
    $foo = $_SESSION['usr_ses_id']; // undefined notice
}
var_dump($foo);                     // NULL
var_dump($_SESSION);                // still undefined undefined + NULL

you will see that it outputs:

Notice: Undefined variable: _SESSION in /code/Kx2RMF on line 2
NULL 
bool(false)
Notice: Undefined variable: _SESSION in /code/Kx2RMF on line 5
NULL
Notice: Undefined variable: _SESSION in /code/Kx2RMF on line 8
NULL 

So unless I am missing something, I’d say the code in that chat script makes no sense.