title: validation of variables with isset
body: <p>So I'm reading this book and learning PHP. In one of the exercises I had to write a form containing a couple of hidden values, then write a "handler" that checks whether these values have been initialized.</p>
  
  <p>The code looks like this:</p>
  
  <pre><code>    $time = (!isset($_POST["time"])) ? NULL : $_POST["time"];
      $user = (!isset($_POST["user"])) ? NULL : $_POST["user"];
  </code></pre>
  
  <p>My question is, is there any reason why the author suggests <code>!isset</code>, as opposed to straightup <code>isset</code>? Why not do it like below?</p>
  
  <pre><code>    $time = (isset($_POST["time"])) ? $_POST["time"] : NULL;
      $user = (isset($_POST["user"])) ? $_POST["user"] : NULL;
  </code></pre>
  
  <p>Thanks! Any pros / cons of either method? </p>