isset($_SESSION['id']) is not working properly : <p>I am trying to use the condition:</p>

<blockquote>
  <p>isset($_SESSION['id'])</p>
</blockquote>

<p>So i just tried </p>

<blockquote>
  <p>echo isset($_SESSION['id']); and echo $_SESSION['id'];</p>
</blockquote>

<p>But this gives output different. In fact true result gives by </p>

<blockquote>
  <p>echo $_SESSION['id'];</p>
</blockquote>

<p>Any one have idea why this is happening?
<br /><strong>Note:</strong> I use <code>session_start();</code> at the beginning of page.</p>
