title: Difference between `if (isset($_SESSION))` and `if ($_SESSION)`?
body: <p>I've noticed that frequently people simply write </p>
  
  <pre><code>&lt;?php  if($_SESSION['username']) {...} ?&gt;
  </code></pre>
  
  <p>while I have been using:</p>
  
  <pre><code> &lt;?php if(isset($_SESSION['username'])) {...} ?&gt; 
  </code></pre>
  
  <p>Could someone explain the difference when checking if a variable is set (that's what I'd be using it for)?</p>