PHP5.6 - 错误访问未声明的静态属性

Question

In my project I get this error:

Access to undeclared static property.

The code apparently is correct:

<?php
class Session {
    private static $sessionStarted = false;

    public static function init()
    {
        if (session_status() !== PHP_SESSION_ACTIVE) {
            session_start();
        }

        if (session_status() === PHP_SESSION_ACTIVE) {
            self::$sessionStarted = true; // <-- this line causes the error!!!
        }
    }

My environment: Windows 10, Windows Bash and Docker php:5.6-apache