此Csrf脚本有时会返回$ hash null

Question

这是我的csrf脚本。它有时会将散列返回为null，从而导致我的Feed无法显示。我用ajax请求feed并通过它发送令牌。现在，任何人都可以帮助弄清楚为什么这只会不时地返回null？我只是没有在这里看到这个问题吗？

    /* Cross-site request forgery attack prevention */
    class CsrfProtection {
        private $previous_hash;
        public  $hash;

        public function __construct() {

            /* Generate a new token */
            $token = md5(time() + time());

            /* Save the previous hash, if there is none then add the new token */
            $this->previous_hash = (isset($_SESSION['token'])) ? $_SESSION['token'] : $token;

            /* Save the new session / variable */
            if(basename($_SERVER['SCRIPT_NAME']) == 'index.php') $_SESSION['token'] = $token;
            @$this->hash = $_SESSION['token'];

        }

        /* Check if post/get iprevious token is valid when sending data to the server */
        public function is_valid($key = 'token') {

            return (isset($_POST[$key]) && ($_POST[$key] === $this->previous_hash)) || (isset($_GET[$key]) && ($_GET[$key] === $this->previous_hash));

        }
    }

每次我在页面上调用时，都使用$ token = new CsrfProtection（）;