Can't solve "header have already been sent" error
<p>I have installed <a href="http://flarum.org/" rel="nofollow">Flarum</a> and tried to create my first extension. I have followed the tutorials and "required" my extension through the root folder's composer.json and ran "composer update". It popped up in my Admin Dashboard of Flarum and as soon as I enabled it I got this error throughout the whole site:</p>

<blockquote>
  <p>RuntimeException Failed to start the session because headers have
  already been >sent by
  "/srv/users/serverpilot/apps/myapplication/public/workbench/flarum-ext-yogi/bootstrap.php" at line 4.</p>
</blockquote>

<p>Extension folder contains only two files: composer.json and bootstrap.php</p>

<p>This is the content of bootstrap.php:</p>

<pre><code>&lt;?php return function () { echo 'Hello world!'; };
</code></pre>

<p>Here's my callstack information: <a href="http://i.imgur.com/myayCyt.png" rel="nofollow">http://i.imgur.com/myayCyt.png</a></p>

<p>As soon as I remove the "echo" from the bootstrap.php my site starts working again. I tried cleaning my cookies and cache but it didn't help.</p>

<p>What might be the reason for this? Why would a simple echo break the whole site?</p>