用户输入的用户名密码进行验证，为空或两次输入密码不一致返回到登录页面（这里也可以自己加写其他验证，如密码长度验证），符合验证条件，进行数据库连接，数据库连接正确如果查询数据库用户名已存在则打印“用户已存在！”返回登录页面，插入失败打印“注册失败！返回登录页面”；插入成功打印“注册成功！返回登录页面“
代码如下：
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>";
}
}