/**
 * @method 生成由大小字母+数字随机字符串
 * @param $length
 * @return null|string
 */
function getRandomChar($length)
{
    $str = null;
    $strPol = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz';
    $max = strlen($strPol) - 1;
    for ($i = 0; $i < $length; $i++) {
        $str .= $strPol[rand(0, $max)];
    }
    return $str;
}
/**
 * @method 生成随机数字
 * @param $length
 * @return string
 */
function getRandomNumber($length)
{
    return substr(str_shuffle(time()), 0, $length);
}