当用户登录时重新登录网站

Question

如果用户想要再次填写登录表单并在用户当前登录时将其提交，会发生什么？ login.php如下：

<?php
    session_start();
    if($_SERVER["REQUEST_METHOD"] == "POST") {
        $username = $_POST['username'];
        $password = $_POST['password'];
        //check fields and if they are exist
        $_SESSION['login_user'] = $username;
        //redirect to profile.php.
        else {//the username or the password is incorrect.}
    }
    else {mysql_close($connection);}