Php effectively prevents multiple logins by the same user

This article mainly introduces how php effectively prevents multiple logins of the same account at the same time. if you are interested, refer [Problem description ]:It is dangerous for a single user to log on multiple times at the same time. Because, you cannot know if other users are logging on to your account. How Can I disable multiple logins from the same user?
[Solution]
(1) A new session_id is generated after the identity authentication is successful upon each login.

session_regenerate_id(); session_register ("username") ; 

(2) open a sessionid field in the user database. after the session_id is re-generated, the field is updated.

$sessionid = session_id(); $db = new PDO('sqlite:softToken.db'); $sql = "update userinfo set sessionid ='$sessionid' where username='$username' and passwd='$passwd';"; $query = $db->prepare($sql); $query->execute(); 

(3) Create a session to save the user name

$_SESSION["username"] = $username; 

(4) use url rewriting to pass session_id

$ Url = "main. php? Sid = ". session_id (); unset ($ db); echo" login successful, jump! "; Header (" Location: $ url ");

(5) add
Main. php

<? Php header ('content-type: text/html; charset = utf-8 '); $ sessionid = $ _ GET ['Sid']; session_id ($ sessionid); session_start (); $ username = $ _ SESSION ["username"]; $ db = new PDO ('sqlite: softToken. db'); $ SQL = "select * from userinfo where username = '$ username' and sessionid =' $ sessionid ';"; $ query = $ db-> prepare ($ SQL); $ query-> execute (); $ user = $ query-> fetch (PDO: FETCH_OBJ ); if ($ user-> username = "") {session_destroy (); echo"