title: $_SERVER['HTTP_HOST'] not set
body: <p>I am getting lot of traffic to my php pages but without the variable <code>$_SERVER['HTTP_HOST']</code> set.</p>
  
  <p>This traffic is like 1 hit per second. I don't know what it could be, but for reducing server load i am doing this at the top of every php pages:</p>
  
  <pre><code>if (!isset($_SERVER['HTTP_HOST']))
      exit;
  </code></pre>
  
  <p>Do you know what could cause this? Is it safe to exit whenever <code>HTTP_HOST</code> is not set? </p>
  
  <p>Can a normal user visit my pages without setting <code>HTTP_HOST</code>?</p>
  
  <ul>
  <li>PHP version: 5.2.0-8</li>
  <li>Apache: 2.2.3</li>
  </ul>