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

function my_session_start()
{
    $sn = session_name();

    if (isset($_COOKIE[$sn])) {
        $sessid = $_COOKIE[$sn];
    } else if (isset($_GET[$sn])) {
        $sessid = $_GET[$sn];
    } else {
        return session_start();
    }

    if (!preg_match('/^[a-zA-Z0-9,\-]{22,40}$/', $sessid)) {
        return false;
    }

    return session_start();
}

if ( !my_session_start() ) {
    session_id( uniqid() );
    session_start();
    session_regenerate_id();
}

參考來源:
http://stackoverflow.com/questions/3185779/the-session-id-is-too-long-or-contains-illegal-characters-valid-characters-are