title: wordpress page template requre_once() failed
body: <p>I want to use a class file in a WordPress page template, but after adding the <code>require_once()</code> method, and trying to instance the class, I got</p>
  
  <p><code>Fatal error: Class 'ClassName' not found</code>, </p>
  
  <p>the class file is displayed as html to the output page.</p>
  
  <p>Any idea why this is happening? (I put the page template file and the class file under the theme directory)</p>