title: Why do I need the isset() function in php?
body: <p>I am trying to understand the difference between this:</p>
  
  <pre><code>if (isset($_POST['Submit'])) { 
    //do something
  }
  </code></pre>
  
  <p>and</p>
  
  <pre><code>if ($_POST['Submit']) { 
    //do something
  }
  </code></pre>
  
  <p>It seems to me that if the $_POST['Submit'] variable is true, then it is set. Why would I need the isset() function in this case?</p>