title: foreach($_SESSION AS $value){...} -> get array key of session
body: <p>I fill a session like this:</p>
  
  <pre><code>$_SESSION[$id]=$value;
  </code></pre>
  
  <p>And I'm reading out the array with this:</p>
  
  <pre><code>foreach($_SESSION AS $value){...}
  </code></pre>
  
  <p>But how can I read out the <code>$id</code> of the session too? Array key?</p>
  
  <p>Thanks!</p>