首先用session将随机生成的验证码的值传到页面，然后获取当前文本框中输入的值  进行对比；代码如下：

生成的随机数，把它传到session里面

<?

session_start();   必须要先打开session开关。
$s = array("a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","0","1","2","3","4","5","6","7","8","9");
$r="";

for($i=1;$i<=4;$i++){
$r .= $s[rand(0,count($s)-1)];
}
$_SESSION[‘nb‘] = $r;

?>

然后写Ajax代码如下：

function checkYZM(){
getXmlhttp();
xmlHttp.open("GET","chekYZM.php?YZM="+yzm.value,"false");
xmlHttp.onreadystatechange = function(){
if(xmlHttp.readyState ==4){
if(xmlHttp.status == 200){
var txt = parseInt(xmlHttp.responseText);
if(txt){
tS5.innerHTML = "验证成功";
}else{
tS5.innerHTML = "验证失败";
}
}
}
}
xmlHttp.send(null);
}

最后在chekYZM.php页面里进行判断：

<?php
session_start();

if(isset($_GET[‘YZM‘])){
$yzm = $_GET[‘YZM‘];
if($yzm == $_SESSION[‘nb‘]){
echo"1";
}else{
echo"0";
}
}
?>