Change to is_null, but the function of preventing repeated login cannot be implemented
Single sign-in principle: //m.sbmmt.com/php-weizijiaocheng-263961.html
Repeated login The most commonly used method is single login
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
Single sign-in principle: //m.sbmmt.com/php-weizijiaocheng-263961.html
Repeated login The most commonly used method is single login