PHP development small forum tutorial - login to PHP page

This page is to compare the data we passed from the login page with the data in the database

If it is incorrect, the user will not be allowed to log in

The specific code is as follows

<?php
session_start();
header("Content-type:text/html;charset=utf-8");
$link = mysqli_connect('localhost','root','root','mybbs');//链接数据库
mysqli_set_charset($link,'utf8'); //设定字符集
$username=$_POST['username'];
$password=md5($_POST['password']);
    if($username==''){
        echo "<script>console.log('请输入用户名');location='" . $_SERVER['HTTP_REFERER'] . "'</script>";
        exit;
    }
    if($password==''){
        echo "<script>console.log('请输入密码');location='" . $_SERVER['HTTP_REFERER'] . "'</script>";
        exit;
    }
$sql="select id,username,password from member where username= $username";      //从数据库查询信息
$que=mysqli_query($link,$sql);
$row=mysqli_fetch_array($que);
if($row){
    if($password !=($row['password']) || $username !=$row['username']){
        echo "<script>console.log('密码错误，请重新输入');location='login.html'</script>";
        exit;
    }
    else{
        $_SESSION['username']=$row['username'];
        $_SESSION['id']=$row['id'];
        echo "<script>console.log('登录成功');location='index.php'</script>";
    }
}else{
    echo "<script>console.log('您输入的用户名不存在');location='login.html'</script>";
    exit;
};
?>

Because we have not registered yet, we cannot log in. Let’s create our registration page