Login processing page

The code is as follows:

<?php  
//登錄處理界面 logincheck.php
//判斷是否按下提交按鈕
    if(isset($_POST["submit"]) && $_POST["submit"] == "登陸")  
    {  
    //將用戶名和密碼存入變量中，供后續(xù)使用
        $user = $_POST["username"];  
        $psw = $_POST["userpwd"];  
        if($user == "" || $psw == "")  
        {
        //用戶名或者密碼其中之一為空，則彈出對(duì)話框，確定后返回當(dāng)前頁的上一頁  
            echo "<script>alert('請(qǐng)輸入用戶名或密碼！'); history.go(-1);</script>";  
        }  
        else  
        {  //確認(rèn)用戶名密碼不為空，則連接數(shù)據(jù)庫
            $conn = mysqli_connect("localhost","root","root");//數(shù)據(jù)庫帳號(hào)密碼為安裝數(shù)據(jù)庫時(shí)設(shè)置
             if(mysqli_errno($conn)){
                echo mysqli_errno($conn);
                exit;
             }
            mysqli_select_db($conn,"userdb");  
            mysqli_set_charset($conn,'utf8'); 
            $sql = "select username,userpwd from user where username = '$user' and userpwd = '$psw'";  
            $result = mysqli_query($conn,$sql);  
            $num = mysqli_num_rows($result);  
            if($num)  
            {  
                echo "<script>alert('成功登錄'); window.location.href='index.php';</script>";  
            }  
            else  
            {  
                echo "<script>alert('用戶名或密碼不正確！');history.go(-1);</script>";  
            }  
        }  
    }  
    else  
    {  
        echo "<script>alert('提交未成功！');</script>";  
    }  
  
?>

Login page program flow chart