php实现Ajax带有验证码的登陆注册功能

<?php session_start(); $salt = 'left'; $username = mysql_real_escape_string($_POST["username"]); $password = mysql_real_escape_string(md5($salt.$_POST['password'])); $checkNum = $_POST["checkNum"]; $feedback = "注册失败"; if($checkNum==$_SESSION["validcode"]){ $con=mysql_connect("localhost:3306","",""); if(!$con){ die('Clould not connect:'.mysql_errno()); } mysql_select_db("mysql",$con); $SQL="select * from login where username='$username'"; $result=mysql_query($SQL); $rows=mysql_num_rows($result); if($rows>=1){ $feedback="该用户已经存在"; } else{ $sql_insert = "insert into login (username,password) values ("."'$username',"."'$password')" ; $res_insert = mysql_query($sql_insert); if($res_insert) { $feedback="注册成功"; } else { $feedback="注册失败"; } } } echo $feedback; mysql_close(); ?>