title: Check if php session string is valid and exists
body: <p>Basically I want something like</p>
  
  <pre><code>$string = "dadeadad";
  
  if(session_id($string)){
        // $string is a valid php session, not expired
  }else{
        // $string is not a valid php session
  }
  </code></pre>
  
  <p>But all I found is examples related to checking if a session was started (that is not what I want). Any idea on how to do this?</p>