code: protected static function createIV($size)
      {
          if (function_exists('openssl_random_pseudo_bytes')) {
              $bytes = openssl_random_pseudo_bytes($size, $strong);
          }
          if (is_null($bytes) || $bytes === false || $strong === false) {
              $size *= 2;
              $pool  = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
              $bytes = pack('H*', substr(str_shuffle(str_repeat($pool, $size)), 0, $size));
          }
          return $bytes;
      }
docs: 随机生成指定长度的初始化向量
  
  @param  int $size 初始化向量长度
  
  @return string