Php生成6位无规律、唯一、不重复的邀请码

//输入当前的用户编号自增长的id 
function getInviteCode($userId)
{
    $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
    $num = strlen($chars);
    $str = '';
    while ($userId > 0) {
        $mod = $userId % $num;
        $userId = ($userId - $mod) / $num;
        $str = $chars[$mod] . $str;
    }

    $cd=$this->createNonceStr(6 - strlen($str));

    // 不足用随机字符串补充，10表示邀请码邀请10位
    $str = str_pad($str, 6, $cd, STR_PAD_LEFT);
    return $str;
}

function createNonceStr($length = 16)
{
    $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
    $str = '';
    for ($i = 0; $i < $length; $i  ) {
        $str .= substr($chars, mt_rand(0, strlen($chars) - 1), 1);
    }
    return $str;
}

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