สอบถามการเขียนเว็บเชื่อมกับ Data base หน่อยครับ พรีสส

/// <reference path="../../assets/js/jquery.d.ts" />

$(document).ready(main);

function main() {

    $(document.body).load("View/main.html", "", function() {

        $.ajax("Model/request.php?req=0", {
            dataType: "json",
            success: function(data) {
                if (data.state == 0) { // state = 0 : load login module
                    login_work();
                } else { // state != 0 : load logout module
                    logout_work(data.username);
                }   
            }
        });

    });
    
}

function login_work() {
    $("#content_area").html("");
    $("#authen_area").html("");
    $("#authen_area").load("View/module_login.html", "", function() {

        $("#btn_do_login").click(function() {

            var username = $("#txt_username").val();
            var password = $("#txt_password").val();

            $.ajax("Model/request.php?req=1", {
                data: {"username":username, "password":password},
                dataType: "json",
                method: "post",
                success: function(data) {
                    if (data.status == true) { // login success
                        logout_work(data.username);
                    } else { // login failed
                        alert("LOGIN FAILED : username OR password INVALID!");
                    }
                }
            });

        });

    });
}

function logout_work(username) {
    $("#authen_area").html(""); // clear all contents in #authen_area
    $("#authen_area").load("View/module_logout.html", "", function() {

        $("#username").html(username);
        $("#btn_logout").click(function() {
            $.ajax("Model/request.php?req=2", {
                success: function() {
                    login_work();
                }
            });
        });

        // call user_work()
        users_work();

    });
}

function users_work(){
    $("#content_area").load("View/module_users.html", function() {

        update_users();

        $("#btn_add").click(function() {
            var username = $("#new_username").val();
            var password = $("#new_password").val();
            console.log (username,password);
            $.ajax("Model/request.php?req=3",{
                data: {"username":username,"password":password},
                method: "post",
                dataType: "json",
                success: function(data) {
                    if (data.status == true){
                       $("#new_username").val("");
                       $("#new_password").val("");
                       // update table 
                       update_users();
                    } else {
                       alert(data.message);
                    }
                }
            });
        });
    });
    function update_users(){

     $.ajax("Model/request.php?req=4", {
        dataType: "json",
        success: function(data) {
           console.log(data);
           if (data.status == true) {
               var tbody = $("#table_body");
               tbody.html (""); // เคลียร์ข้อมูลเก่า
               for (var i = 0; i < data.data.length; i++) {
                   tbody.append(
                       "<tr>"+
                       "<td>"+data.data[i][0]+"</td>"+
                       "<td>"+data.data[i][1]+"</td>"+
                       "<td>********</td>"+
                       "<td><button>X</button></td>"+
                       "</tr>"
                    );
                    
                }
            }
       }
     });
      

    }
    
}

<?php

session_start();

include_once("db.php");

$output = array();

$req = (int)$_REQUEST["req"];

if ($req == 0) {
    $_SESSION["state"] = (empty($_SESSION["state"]))? 0 : $_SESSION["state"];
    $output["state"] = $_SESSION["state"];
    $output["username"] = $_SESSION["username"];
}
// req = 1 : login
else if ($req == 1) {
    $username = $_REQUEST["username"];
    $password = $_REQUEST["password"];
    if (get_user($username, $password) == $username) { // login success
        $_SESSION["state"] = 1;
        $_SESSION["username"] = $username;
        $output["status"] = true;
        $output["username"] = $username;
    } else { // login failed
        $output["status"] = false;
    }
}
// req = 2 : logout
else if ($req == 2) {
    $_SESSION["state"] = 0;
}
// req = 3 : add users
else if ($req == 3) {
    $username = $_REQUEST["username"];
    $password = $_REQUEST["password"];
    $result = add_user($username, $password);
    if ($result = "succes") {
        $output ["status"] = true;
    } else {
        $output ["status"] = false;
        $output ["message"] = $result;
    }
}
// req = 4 : get users data 
else if ($req == 4) {
   $data = read_users($_SESSION["username"]);
   if ($data) {
       $output ["status"] = true;
       $output ["data"] = $data;
       
   } else {
       $output ["status"] = false;
   }
}
echo json_encode($output);

?>

<?php

define("DB_HOST", "localhost");
define("DB_USERNAME", "root");
define("DB_PASSWORD", "1234"); // 1234
define("DB_NAME", "mvc_sample_03");
define("DB_CHARSET", "utf8");

$linker = mysql_pconnect(DB_HOST, DB_USERNAME, DB_PASSWORD);

if (mysql_errno($linker) != 0) {
    echo mysql_error($linker);
    exit(0);
}

mysql_set_charset(DB_CHARSET, $linker);
mysql_select_db(DB_NAME, $linker);

function get_user($username, $password) {
    global $linker;

    //$password = md5($password);

    $sql = "SELECT * FROM users WHERE username = '$username' AND password = PASSWORD('$password');";
    $result = mysql_query($sql, $linker);

    if ($result) {
        if (mysql_num_rows($result) > 0) {
            $row = mysql_fetch_row($result);
            return $row[1];
        }
    } else {
        echo $sql;
    }

    return "";
}

function add_user ($username, $password) {
    global $linker;
    $sql = "INSERT INTO users (username, password) VALUES('$username',PASSWORD('$password'));";
    $result = mysql_query($sql,$linker);

  if (mysql_errno() == 1062){
     return "Duplicate Username \"$username\"";
  } 

   return "success";
}

function read_users ($username) {
    global $linker;
    $sql = "SELECT id, username FROM users WHERE username NOT IN ('admin','$username');";
    $result = mysql_query($sql, $linker);
    if ($result) {
        if (mysql_num_rows($result) > 0 ) {
            $records = array();
            while ($row = mysql_fetch_row($result)) {
               $records[count($records)] = $row;
            }
            return $records;
        }
    }
    return null;
}

?>