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('用戶名不能為空！');location.href='login.html';</script>";
    }else {
        $username = trim($_POST['username']);
    }
    if (empty($_POST['password'])){
        echo "<script>alert('密碼不能為空！');location.href='login.html';</script>";
    }else{
        $password = $_POST['password'];
    }
    if (empty($_POST['repassword'])){
        echo "<script>alert('確認密碼不能為空！');location.href='login.html';</script>";
    }else{
        $repassword = $_POST['repassword'];
    }
    if ($password != $repassword) {
        echo "<script>alert('兩次輸入密碼不一致！');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('用戶已存在！');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('注冊失敗！');location.href='login.html';</script>";
    } else {
        echo "<script>alert('注冊成功！返回登錄頁面');location.href='login.html';</script>";
    }
}