How do PHP and MySql implement reading of background data? (Code)

<?php
    header("Content-Type:text/html;charset=utf-8");
    
    
    define("HOST", "127.0.0.1");
    define("USERNAME", "root");
    define("PASSWORD", "");
    define("DBNAME", "mydb");
    define("CHARSET", "utf8");
    
    $con=mysqli_connect(HOST, USERNAME, PASSWORD, DBNAME) or die("数据库连接失败，<span style=&#39;color:red;&#39;>".mysqli_connect_error()."</span>");
    mysqli_set_charset($con, CHARSET) or die("字符集编码设置无效");

<p class="panel panel-primary">
            <p class="panel-heading">
                <p class="panel-title">用户注册</p>
            </p>
            <p class="panel-body">
                <form class="form-horizontal">
                    <p class="form-group">
                        <label>用户名</label>
                        <input type="text" class="form-control" name="userName"/>
                    </p>
                    <p class="form-group">
                        <label>密码</label>
                        <input type="password" class="form-control" name="pwd" />
                    </p>
                    <p class="form-group">
                        <label>确认密码</label>
                        <input type="password" class="form-control" name="rePwd" />
                    </p>
                    <p class="form-group">
                        <label>真实姓名</label>
                        <input type="text" class="form-control" name="realName" />
                    </p>
                    
                    <p class="form-group btns">
                        <input type="button" class="btn btn-primary" value="确定注册" id="submit"/>
                            
                        <a type="button" class="btn btn-success" href="login.php"/>返回登录</a>
                    </p>
                    
                </form>
            </p>
        </p>

<script src="../../js/jquery-1.10.2.js"></script>
    <script type="text/javascript">
        $(function(){
            $("#submit").on("click",function(){
                var userName = $("input[name=&#39;userName&#39;]").val();
                var pwd = $("input[name=&#39;pwd&#39;]").val();
                var rePwd = $("input[name=&#39;rePwd&#39;]").val();
                var realName = $("input[name=&#39;realName&#39;]").val();
                if(userName==""||pwd==""||rePwd==""||realName==""){
                    alert("所有信息不可为空，请确认！");
                    return;
                }else if(pwd!=rePwd){
                    alert("两次密码输入不一致！");
                    return;
                }
                
                $.post("doReg.php",{
                    "userName":userName,
                    "pwd":pwd,
                    "realName":realName
                },function(data){
                    alert(data);
                    
                    if(data=="注册成功"){
                        location = "login.php";
                    }
                })
                
            });
        });
    </script>

<?php
    header("Content-Type:text/html;charset=utf-8");
    $str=$_POST["formData"];
    list($username)=explode("&", $str);
    list(,$pwd)=explode("&", $str);
    list(,,,$realname)=explode("&", $str);
    list(,$username)=explode("=", $username);
    list(,$pwd)=explode("=", $pwd);
    list(,$realname)=explode("=", $realname);
    include_once("mysql.php");
    $sql=<<<sql
    insert into submit (username,pwd,realname) values ("$username","$pwd","$realname");
sql;
    $sql2=<<<sql2
    SELECT username FROM submit WHERE username="$username";
sql2;
    $res2=mysqli_query($con, $sql2);
    $res=mysqli_query($con, $sql);
    if(mysqli_num_rows($res2)>0){
        die("用户名已经存在！");  
    }
    elseif($res){
        echo &#39;true&#39;;
    }else{
        die();
    }

<p class="panel panel-primary">
            <p class="panel-heading">
                <p class="panel-title">用户登录</p>
            </p>
            <p class="panel-body">
                <form class="form-horizontal">
                    <p class="form-group">
                        <label>用户名</label>
                        <input type="text" class="form-control" name="userName"/>
                    </p>
                    <p class="form-group">
                        <label>密码</label>
                        <input type="password" class="form-control" name="pwd"/>
                    </p>
                    
                    <p class="form-group btns">
                        <input type="button" class="btn btn-primary" value="登录系统" id="submit"/>
                            
                        <a type="button" class="btn btn-success" href="reg.php"/>注册账号</a>
                    </p>
                    
                </form>
            </p>
        </p>

<script src="../../js/jquery-1.10.2.js"></script>
    <script type="text/javascript">
        $(function(){
            $("#submit").on("click",function(){
                var userName = $("input[name=&#39;userName&#39;]").val();
                var pwd = $("input[name=&#39;pwd&#39;]").val();
                
                $.post("doLogin.php",{
                    "userName":userName,
                    "pwd":pwd
                },function(data){
                    alert(data);
                    if(data=="登录成功"){
                        location = "index.php";
                    }else{
                        alert("用户名或密码有误！");
                    }
                });
            });
        });
    </script>

<?php

    header("Content-Type:text/html;charset=utf-8");include_once("../mysql/mysql.php");
    
    $userName = $_POST["userName"];
    $pwd = $_POST["pwd"];
    
    $loginSql = <<<login
    select * from submit where username="{$userName}" and pwd = "{$pwd}";
login;

    $res = mysqli_query($con, $loginSql);
    
    if($row = mysqli_fetch_row($res)){
        $_SESSION["user"] = $row;

        echo "登录成功";
    }else{
        echo "登录失败";
    }

    
    mysqli_free_result($res);
    mysqli_close($con);