title: Symfony2: The session id is too long or contains illegal characters
body: <p>I always get this error when I have some time without updating a project, only occurs in the development environment.</p>
  
  <pre><code>Warning: session_start() [function.session-start]: The session id contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,'
  </code></pre>
  
  <p>How to avoid? or Why does this happen?</p>
  
  <p>i use:</p>
  
  <ul>
  <li>Symfony 2.0.9</li>
  <li>Linux Mint 12 3.0.0-14-generic x86_64</li>
  <li>PHP 5.3.6-13ubuntu3.3</li>
  </ul>