title: What is $previous in Exception class?
body: <p>What does <code>$previous</code> in the <code>Exception</code> constructor argument denote? How could I use it?</p>
  
  <pre><code>    class MyException extends \Exception {
  
      private $message;
      private $code;
  
  
      public function __construct($message,$code,\Exception $previous=null){
          $this-&gt;message = $message;
          $this-&gt;code = isset($code) ? $code : 0;
          parent::__construct($message,$code,$previous);      
      }       
  }
  </code></pre>
  
  <p>I didn't find anything in the <a href="http://php.net/manual/en/class.exception.php" rel="nofollow">API Doc</a></p>