怎么使用php实现用户登录

  <link rel="stylesheet" href="css/bootstrap.css">
 	<style>
          li.toggle{
              display: block;
           }
          li.toggle1{
              display: none;
            }
        </style>
        <li class="toggle"><a data-toggle="modal" href="#loginer">登录</a></li>
        <li class="toggle"><a data-toggle="modal" href="#register">注册</a></li>
        <li class="toggle1"><a id="nicheng" href="##"></a></li>
        <li class="toggle1"><a id="logout" href="##">注销</a></li>
//模态框
<p id="loginer" class="modal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel1"
     aria-hidden="true">
    <p class="modal-dialog">
        <p class="modal-content">
            <p class="modal-header">
                <button type="button" class="close" data-dismiss="modal">
                    <span aria-hidden="true">&times;</span><span class="sr-only">Close</span></button>
                <h4 class="modal-title">请登录</h4>
            </p>
            <form id="form_login" class="form-horizontal">
                <p class="modal-body">
                    <p class="form-group has-feedback">
                        <label class="col-sm-3 control-label" for="zh1">用户名：</label>
                        <p class="col-sm-9 require">
                            <input type="text" class="form-control" name="username" id="zh1"
                                   pattern="\w{6,10}" required>
                        </p>
                    </p>
                    <p class="form-group has-feedback">
                        <label class="col-sm-3 control-label" for="pw1">密码：</label>
                        <p class="col-sm-9 require">
                            <input type="password" pattern="\w{6,10}" class="form-control" name="pwd" id="pw1"
                                   required>
                        </p>
                    </p>
                    <p class="form-group has-feedback">
                        <label class="col-sm-3 control-label" for="yzm">验证码：</label>
                        <p class="col-sm-5 require">
                            <input type="text" pattern="\d{4}" class="form-control" name="yzm" id="yzm"
                                   required>
                        </p>
                        <p class="col-sm-4">
                            @@##@@
                        </p>
                    </p>
                    <p class="form-group has-feedback">
                        <label id="info" class="col-sm-4 control-label pull-left"></label>
                    </p>
                </p>
                <p class="modal-footer">
                    <button type="submit" class="btn btn-primary">登录</button>
                    <button type="button" class="btn btn-default" data-dismiss="modal">关闭</button>
                </p>
            </form>
        </p>
    </p>
</p>

<script src="js/jquery-2.1.3.js"></script>
<script src="js/bootstrap.min.js"></script>
<script>
    function toggle(){
        $.getJSON("php/get.php",function (res) {
            if(res['flag']){
                $('li.toggle').hide();
                $('li.toggle1').show();
                $('#nicheng').html(res['nc'])
            }else {
                $('li.toggle').show();
                $('li.toggle1').hide();
            }
        });
    }
    toggle();
    $('#form_login').submit(function (e) {
        e.preventDefault();
        var data=$('#form_login').serialize();
        $.getJSON('php/login.php',data,function (res) {
            if(res==3){
                toggle();
                $('#loginer').modal('hide');
            }else if(res==2){
                $('#info').html('用户名或密码有误')
            }else {
                $('#info').html('验证码有误')
            }
        })
    });
    $('#logout').click(function () {
        confirm('确定要注销？');
        $.getJSON('php/logout.php',function (res) {
            if(res)toggle();
        })
    })
    </script>

<?php
$link=new PDO("mysql:host=localhost;port=3306;dbname=db","root","");
$link->query("set names utf8");

<?php
include_once ("connect.php");
session_start();/*开启会话*/
if (isset($_SESSION['username'])){
   /* 判断用户会话里用户名是否存在，即用户是否登录*/
    $json['nc']=$_SESSION['nc'];/*把昵称存起来，一会返回给首页*/
    $json['flag']=true;/*用户已经登录，标志flag为true*/
}else
    $json['flag']=false;/*用户已经登录，标志flag为true*/
echo json_encode($json);/*返回json*/

<?php
include_once ("connect.php");
session_start();/*开启会话*/
$user=$_GET['username'];/*获取登录表单提交过来的数据*/
$pwd=$_GET['pwd'];
$yzm=$_GET['yzm'];
if($yzm==$_SESSION['vCode']){/*当用户输入的验证码和图片验证码相同时*/
    $result=$link->query("select * from `user` where username='$user' and pwd='$pwd'");
    $link = null;
    $row = $result->fetch();/*读取从数据库获取的数据*/
    if ($row) {/*如果数据存在，即用户登录成功*/
        $_SESSION['username'] = $row['username'];
        /*将用户名和昵称存在服务器，可以多个页面使用*/
        $_SESSION['nc'] = $row['nc'];
        $flag=3;
    }else{/*用户名或密码错误*/
        $flag=2;
    }
}else{/*验证码输入错误*/
    $flag=1;
}
echo $flag;

<?php
session_start();
unset($_SESSION['username']);//销毁用户名
unset($_SESSION['nc']);//销毁昵称
echo json_encode(true);//返回结果