code: private function getFileData($fileName)
      {
          if (! is_file($fileName)) {
              return null;
          }
          $createTime = filectime($fileName);
          $data = json_decode(file_get_contents($fileName), true);
          //判断是否过期
          if ($data['expire'] != '0' && ($data['expire'] + $createTime) < time()) {
              unlink($fileName);
              return null;
          }
          return $data['data'];
      }
docs: 获得文件内容
  @param $fileName