title: Undefined index HTTP_HOST even though it is checked
body: <p>Here's the code:</p>
  
  <pre><code>if (isset($_SERVER['HTTP_HOST']) === TRUE) {
    $host = $_SERVER['HTTP_HOST'];
  }
  </code></pre>
  
  <p>How is it possible to get an "Undefined index HTTP_HOST" on the line inside the if statement? I mean, the index setting is checked before it is used. </p>
  
  <p>And why could the HTTP_HOST sometimes be not set? </p>