code: private function getNumberFromVariation($variationId)
      {
          static $variations;
  
          if (!isset($variations[$variationId])) {
              $response = $this->client->request('GET', 'items/variations', ['id' => $variationId]);
  
              if (empty($response)) {
                  $variations[$variationId] = null;
  
                  return $variations[$variationId];
              }
  
              $variation = array_shift($response);
  
              $variations[$variationId] = $variation['number'];
          }
  
          return $variations[$variationId];
      }
docs: @param int $variationId
  
  @return string