As a novice, I originally imagined that when adding a user, I would select the value of value, enter the username and password when logging in, and at the same time determine whether the value of the selected value is 0 or 1, and enter different pages

The value is saved in the database as class

But I don’t know how to take a certain person’s class value out of the database. Judgment, I don’t know how to improve the code. Please give me some advice.

<meta charset="utf-8">
<?php 
    
    if(isset($_POST["submit"]) && $_POST["submit"] == "登陆") 
    { 
        $user = $_POST["username"]; 
        $psw = $_POST["password"]; 
        
        $Dclass = $_POST["class"]; 
        if($user == "" || $psw == "") 
        { 
            echo "<script>alert('请输入用户名或密码！'); history.go(-1);</script>"; 
        } 
        else 
        { 
            mysql_connect("localhost","root","root"); 
            mysql_select_db("hospital"); 
            mysql_query("set names utf-8"); 
            $sql = "select username,password,ID,class from user where username = '$_POST[username]'and password = '$_POST[password]'"; 
            $result = mysql_query($sql); 
            $num = mysql_num_rows($result); 
            if($num) 
            { 
                $row = mysql_fetch_array($result);  //将数据以索引方式储存在数组中 
                echo $row[0]; 
                
            } 
            else 
            { 
                echo "<script>alert('用户名或密码不正确！');history.go(-1);</script>"; 
            } 
        } 
    } 
    else 
    {       mysql_connect("localhost","root","root"); 
            mysql_select_db("hospital"); 
            mysql_query("set names utf-8"); 
            $sql = " Dclass = '$_POST[class]'"; 
            $result = mysql_query($sql); 
                  

            if($result=="0"){
          //医生的页面
          echo"<script>alert('登录成功哦，您是医生！');location.href='../hospitalmain.html'</script>";
                                     }
        else{
          //管理员的页面
          echo"<script>alert('登录成功哦，您是管理员！');location.href='../adminmain.html'</script>";
            }

        /*echo "<script>alert('登录成功！欢迎回来！');window.location.href='../costmain.html'</script>";*/
   }

?>