title: Is it possible for $_SERVER['HTTP_USER_AGENT'] to not be set?
body: <p>I've just been looking through a website's <code>error_log</code> and one of the error's that has been logged a few times is:</p>
  
  <p><code>[21-Jun-2011 12:24:03] PHP Notice:  Undefined index:  HTTP_USER_AGENT in /home/ukevents/public_html/lib/toro.php on line 130</code></p>
  
  <p>The line this pertains to in <strong>toro.php</strong> is:</p>
  
  <pre><code>private function ipad_request() {
      return strstr($_SERVER['HTTP_USER_AGENT'], 'iPad');
  }
  </code></pre>
  
  <p>Is it possible for <code>$_SERVER['HTTP_USER_AGENT']</code> to not be set by a HTTP request?</p>