php 生成随机邀请码

//随机生成邀请码
function getRandomString($len, $chars=null)
{
    if (is_null($chars)){
        $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    }

    for ($i = 0, $str = '', $lc = strlen($chars)-1; $i < $len; $i++){
        $str .= $chars[mt_rand(0, $lc)];
    }
    return $str;
}

getRandomString(6); //生成6位随机邀请码