<?php

function rand_string($len = 16, $keyword = '') {
    if (strlen($keyword) > $len) {//关键字不能比总长度长
        return false;
    }
    $str = '';
    $chars = 'abcdefghijkmnpqrstuvwxyz23456789ABCDEFGHIJKMNPQRSTUVWXYZ'; //去掉1跟字母l防混淆            
    if ($len > strlen($chars)) {//位数过长重复字符串一定次数
        $chars = str_repeat($chars, ceil($len / strlen($chars)));
    }
    $chars = str_shuffle($chars); //打乱字符串
    $str = substr($chars, 0, $len);
    if (!empty($keyword)) {
        $start = $len - strlen($keyword);
        $str = substr_replace($str, $keyword, mt_rand(0, $start), strlen($keyword)); //从随机位置插入关键字
    }
    return $str;
}

echo rand_string(16,"ab"); //output example:V8bNY6SmkeywordB
?>

?