php 生成随机字符串 abcdeft....789

private function createNonceStr($length = ) {
    $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    $str = "";
    for ($i = ; $i < $length; $i++) {
      $str .= substr($chars, mt_rand(, strlen($chars) - ), );
    }
    return $str;
  }