title: Codeigniter header() error
body: <p>I have run into this problem in my Codeigniter app:</p>
  
  <pre><code>A PHP Error was encountered
  Severity: Warning
  Message: Cannot modify header information - headers already sent by (output started at /application/controllers/training.php:1)
  Filename: libraries/Session.php
  Line Number: 671
  </code></pre>
  
  <p>I have been checking for whitespaces and I am not sending headers.</p>
  
  <p>The ONLY way this error appears is when I am updating the view and controller files.</p>
  
  <p>My code ran fine until I updated some files, then the error (above) refuses to disappear.</p>
  
  <p>When I clear my cache and log back in again, the error doesn't show.</p>
  
  <p>Does anyone know why?</p>