The session id is too long or contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,'

It is an information vulnerability: a malicious attacker may alter the cookies and assign illegal characters to PHPSESSID to expose this PHP warning, which in fact contains juicy information like the file path and the username!

have a look at this session_start() discussion for a work-around:

session_start() generate a warning if PHPSESSID contains illegal characters

Warning: session_start() [function.session-start]: The session id contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,' in /home/para/dev/mon_site/header.php on line 17

To avoid i wrote this :

   <?php
        function my_session_start()
        {
            if (ini_get('session.use_cookies') && isset($_COOKIE['PHPSESSID'])) {
                $sessid = $_COOKIE['PHPSESSID'];
            } elseif (!ini_get('session.use_only_cookies') && isset($_GET['PHPSESSID'])) {
                $sessid = $_GET['PHPSESSID'];
            } else {
                session_start();
                return false;
            }

           if (!preg_match('/^[a-z0-9]{32}$/', $sessid)) {
                return false;
            }
            session_start();

           return true;
        }
    ?>

There is a bug report for this problem (https://bugs.php.net/bug.php?id=68063)

You can check the success of your session_start and generate the id if needed:

$ok = @session_start();
if(!$ok){
session_regenerate_id(true); // replace the Session ID
session_start(); 
}