<?php
    //定义验证码库
    $arr = array_merge(range(‘a‘,‘z‘),range(‘A‘,‘Z‘),range(1,9));
    //将数组中元素的顺序打乱
    shuffle($arr);
    //获取每一个元素的下标
    $keys = array_rand($arr,4);
    //遍历取出验证码
    $captcha = "";
    foreach($keys as $val){
        $captcha .= $arr[$val]; 
    }
    echo $captcha;
?>