Registration verification

The user name and password entered by the user are verified. If it is empty or the password entered twice is inconsistent, the user returns to the login page (you can also add other verifications yourself here, such as password length verification). If the verification conditions are met, connect to the database. , the database connection is correct. If the query database user name already exists, print "User already exists!" and return to the login page. If the insertion fails, print "Registration failed! Return to the login page"; if the insertion is successful, print "Registration successful! Return to the login page"

The code is as follows:

add.php

<?php
/**
 * Created by PhpStorm.
 * User: Administrator
 * Date: 2018/2/27 0027
 * Time: 上午 11:06
 */
header('Content-type:text/html;charset=utf-8');
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    if (empty($_POST['username'])){
        echo "<script>alert('用户名不能为空！');location.href='login.html';</script>";
    }else {
        $username = trim($_POST['username']);
    }
    if (empty($_POST['password'])){
        echo "<script>alert('密码不能为空！');location.href='login.html';</script>";
    }else{
        $password = $_POST['password'];
    }
    if (empty($_POST['repassword'])){
        echo "<script>alert('确认密码不能为空！');location.href='login.html';</script>";
    }else{
        $repassword = $_POST['repassword'];
    }
    if ($password != $repassword) {
        echo "<script>alert('两次输入密码不一致！');location.href='login.html';</script>";
    }
}
$mysqli = new mysqli('localhost', 'root', 'root', 'student');
$result = $mysqli->query("SELECT password FROM user WHERE username = "."'$username'");
$rs=$result->fetch_row();
if(!empty($rs)){
    echo "<script>alert('用户已存在！');location.href='login.html';</script>";
}else {
    $mysqli = new mysqli('localhost', 'root', 'root', 'student');
    $sql = "INSERT INTO user (username,password) VALUES ('$_POST[username]', '$_POST[password]')";
    $rs = $mysqli->query($sql);
    if (!$rs) {
        echo "<script>alert('注册失败！');location.href='login.html';</script>";
    } else {
        echo "<script>alert('注册成功！返回登录页面');location.href='login.html';</script>";
    }
}