title: PHP7 + Symfony 2.8, Failed to write session data
body: <p>i compiled php7 on my own (974f6c2a705).
  if i run php7 + php-fpm + nginx using symfony i get this error:</p>
  
  <p>(using the snc redis bundle for sessions:)</p>
  
  <pre><code> Warning: session_write_close(): Failed to write session data (user). Please verify that the current setting of session.save_path is correct (/tmp)
  </code></pre>
  
  <p>(using the native session support:)</p>
  
  <pre><code> Warning: session_write_close(): Failed to write session data (user). Please verify that the current setting of session.save_path is correct (/[...]/app/cache/dev/sessions)
  </code></pre>
  
  <p>the problem seems to be symfony related, because php has read / write access to the folder.</p>
  
  <p>if i run nothing but this code, it works:</p>
  
  <pre><code>session_start();
  $_SESSION['x'] = 4234;
  session_write_close();
  </code></pre>
  
  <p>any suggestions or ideas why symfony fails to write the sessions?</p>