title: Cannot start session when headers already sent in Participate.php on line 1
body: <p>This is my code in 
  Participate.php
  file:</p>
  
  <pre><code>&lt;?php session_start();
  $_SESSION['id'] = 'Apps';
  ?&gt;
  &lt;!DOCTYPE html&gt;
  &lt;html&gt;
  &lt;head&gt;
  ...
  &lt;/head&gt;
  &lt;body&gt;
   ...
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>But it shows error: </p>
  
  <blockquote>
    <p>Warning: PHP Startup: Cannot start session when headers already sent in /Participate.php on line 1</p>
  </blockquote>
  
  <p>I checked for white spaces there are no any. <strong>session_start();</strong> isn't invoking after header. Html section looks fine. File isn't included somewhere, is separated.</p>