php curl大文件下载

直接分段写入本地文件 ，防止超出内存限制

if (isset($options['CURLOPT_WRITEFUNCTION_PATH'])) {
          $localPath = $options['CURLOPT_WRITEFUNCTION_PATH'];
          $options[CURLOPT_WRITEFUNCTION] = function ($ch, $string) use ($localPath) {
              $fp = fopen($localPath,'a');
              $length = fwrite($fp, $string);
              fclose($fp);
              return $length;
          };
          unset($options['CURLOPT_WRITEFUNCTION_PATH']);
      }