what does this php expression regarding session variables mean?
<p>What does isset($_SESSION) mean? I found the following code snippet-</p>

<pre><code>if (!isset($_SESSION)) {

// php code

}
</code></pre>

<p>EDIT:</p>

<p>I found the following snippet in define.php of the facebook style chatting script 
<a href="http://evnix.com/drupal2/" rel="nofollow">freichat</a> :</p>

<pre><code>if (!isset($_SESSION)) {
            $this-&gt;frm_id = $_SESSION[$this-&gt;uid . 'usr_ses_id'];
            $this-&gt;frm_name = $_SESSION[$this-&gt;uid . 'usr_name'];
        }
</code></pre>

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