生成32位字母数字的随机数

<?php
function create_rand_key($length = 32) 
{
    $chars = 'abcdefghijklmnopqrstuvwxyz0123456789';
    $str = '';
    for ($i = 0; $i < $length; $i++)
    {
        $str .= substr($chars, mt_rand(0, strlen($chars) - 1), 1);
    }
    return $str;
}