Collection::append PHP Method

    public function append($key, $object)
    {
        $this->data = $this->data + array($key => $object);
        return $this;
    }

 /**
  * Scrape URL and collect output
  * @param string $url
  * @param OutputInterface $output
  */
 public function scrape($url, OutputInterface $output)
 {
     $this->collection->exchangeArray([]);
     try {
         $initialPage = $this->client->get($url)->send();
     } catch (BadResponseException $e) {
         $output->writeln('<error>' . $e->getMessage() . '</error>');
         throw new \RuntimeException('Unable to load initial page');
     }
     $xml = $this->getSimpleXml($initialPage->getBody());
     $items = $xml->xpath($this->config['collection']);
     foreach ($items as $item) {
         $title = $item->xpath($this->config['title']);
         $unitPrice = $item->xpath($this->config['unitPrice']);
         $itemUrl = $item->xpath($this->config['itemUrl']);
         $itemSize = 0;
         $itemDesc = null;
         if (isset($itemUrl[0]->attributes()['href'])) {
             try {
                 $itemPage = $this->client->get((string) $itemUrl[0]->attributes()['href'])->send();
                 $itemPageBody = $this->getSimpleXml($itemPage->getBody());
                 $itemSize = $itemPage->getContentLength();
                 $itemDesc = $itemPageBody->xpath($this->config['desc']);
             } catch (BadResponseException $e) {
                 $output->writeln('<error>' . $e->getMessage() . '</error>');
             }
         }
         if ($title && $unitPrice) {
             $parsedPrice = (double) \trim(\str_replace('&pound', null, $unitPrice[0]));
             $this->collection->append(['title' => \trim($title[0]), 'unit_price' => $parsedPrice, 'description' => \trim($itemDesc[0]), 'size' => \round($itemSize / 1024) . 'kb']);
         }
     }
     return $this->collection;
 }