登录时的验证码怎么写？

<div style="margin-left:510px;margin-top:10px;margin-bottom:125px">    
        <div>
            <span  style="color:white; font-weight:bolder; font-size:18px">工号：</span>
            <input type="text" class="dl" id="uid"/>&nbsp;
            <span style="color:white; font-weight:bold; font-size:18px" class="ts" id="uidts"></span>
        </div>
        <br />
        <div>
            <span  style="color:white; font-weight:bolder; font-size:18px">密码：</span>
            <input type="password" class="dl" id="pwd" />&nbsp;
            <span style="color:white;font-weight:bold; font-size:18px" class="ts" id="pwdts"></span>
        </div>
        <br />    
        <div>
            <span style="color:white; font-weight:bolder; font-size:18px;margin-left: -2.5%;">验证码：</span>
            <input type="text" class="dl" id="sryzm"  />&nbsp;
            <span id="ysyzm"  style="color:white; font-weight:bolder; font-size:18px; min-width:70px"></span>&nbsp;&nbsp;&nbsp;
            <input id="sxyzm" type="button" class="btn btn-default" value="刷新" style="font-weight:200" />&nbsp;&nbsp;
            <span style="color:white;font-weight:bold; font-size:18px" class="yzmts" id="yzmts"></span>
            <br /><br />
        </div>
        <div style="clear:both"></div>
        <div  style="margin-left:100px;">
           <input type="button" class="btn btn-default" id="btn" value="登录" style="font-weight:bolder" />
        </div>   
     </div>

var ysyzm = "";
var sryzm = "";
$(document).ready(function(e) {
    
    var str = "qgfh12kjQWFEfEW3EsaeWE544ssgW6GGGgu5mWEE6777EEWW7b67fec";
    var n = 5, s = "";
    for(var i = 0; i < n; i++){
        var rand = Math.floor(Math.random() * str.length);
        
        s += str.charAt(rand);
    }
    $("#ysyzm").html(s);
    ysyzm = s;
    
    $("#sxyzm").click(function(){
                
                s="";
                
                for(var i = 0; i < n; i++){
                var rand = Math.floor(Math.random() * str.length);//指定选取位置
                s += str.charAt(rand);//指定选取字符
                
                /*random() 方法可返回介于 0 ~ 1 之间的一个随机数。
                Math.floor 数学函数，求一个浮点数的地板，就是求一个最接近它的整数，它的值小于或等于这个浮点数。
                charAt(int index)方法是一个能够用来检索特定索引下的字符的String实例的方法.
                charAt()方法返回指定索引位置的char值。索引范围为0~length()-1.
                如: str.charAt(0)检索str中的第一个字符,str.charAt(str.length()-1)检索最后一个字符.*/
                
                }
                $("#ysyzm").html(s);
                ysyzm = s;
        
        });

$("#btn").click(function(){
            sryzm =  $("#sryzm").val();
            //取用户名和密码
            var a = $("#uid").val();
            var b = $("#pwd").val();
            //alert(ysyzm);alert(sryzm);
            var aa = ysyzm.toLowerCase();
            var bb = sryzm.toLowerCase();
            if(aa==bb)
            {
                $.ajax({
                    
                        url:"loginchuli.php",
                        data:{a:a,b:b},
                        type:"POST",
                        dataType:"TEXT",
                        success:function(data){
                                
                                 if(data.trim()=="0")
                                 {
                                    window.location.href="kuangjia.php";
                                 }
                                 else if(data.trim()=="011")
                                 {
                                    window.location.href="../club1/kuangjia.php";
                                 }    
                                 else if(data.trim()=="1")
                                 {
                                     alert(data);
                                    $("#uidts").html(" 请输入用户名！"); 
                                    $(".dl").css("border-color","");
                                    $("#pwdts").html(""); 
                                    $("#uid").css("border-color","#990000");
                                 }    
                                 else if(data.trim()=="2")
                                 {
                                     alert(data);
                                    $("#pwdts").html("    请输入密码！"); 
                                    $(".dl").css("border-color","");
                                    $("#uidts").html(""); 
                                    $("#pwd").css("border-color","black");
                                 }    
                                 else if(data.trim()=="3")
                                 {
                                    alert(data);
                                    $("#uidts").html(" 请输入用户名！"); 
                                    $(".dl").css("border-color","#990000");
                                    $("#pwdts").html("    请输入密码！"); 
                                         
                                 }
                                 else
                                 {
                                     alert(data);
                                     $(".ts").html(" 用户名或密码错误！"); 
                                     $(".dl").css("border-color","black");
                                 }
                                
                            }
                    
                    });
            
            }
            else
            {
                $("#yzmts").html("您输入的验证码有误！");    
            }
            
        })
});

<?php
session_start();

$num = $_POST["a"];
$pwd = $_POST["b"];

$_SESSION["num"]=$num;
include("DBDA.class.php");

$db = new DBDA();

$sql = "select pwd from login where num=‘{$num}‘";
$sql1 = "select code from login where num=‘{$num}‘";

$zwdh = $db->StrQuery($sql1);
$mm = $db->StrQuery($sql);



if($mm == $pwd && $pwd!="")
{
    $_SESSION["zwdh"]=$zwdh;
    echo "0";
    if($num=="n007")    
    {
        echo "11";    
    }
    
}
else if($uid=="" && $pwd=="")
{
    echo "3";
    
}
else if($uid=="" || $pwd=="")
{
    if($uid=="")
    {
        echo "1";
    }
    else if($pwd=="")
    {
        echo "2";        
    }
}
else
{
    echo "4";    
}