PHP Undefined array key warning

I am creating an associative array from XML feed using foreach as you see here:

function load_xml() {
        global $db;

        if(!$xml=simplexml_load_file($this->path)){
            trigger_error('Error reading XML file',E_USER_ERROR);
            }
        foreach ($xml as $item) {
            $xml_items[] = $item_url;
            $data[] = ['item_id' => null,
                       'title' => $item->PRODUCT->__toString(),
                       'title_draft' => null,
                       'description' => $item->DESCRIPTION_SHORT->__toString(),
                       'description_draft' => null,
                       'text' => $item->DESCRIPTION->__toString(),
                       'text_draft' => null,
                       'item_url' => $item->URL->__toString(),
                       'merchant' => $item->MANUFACTURER->__toString(),
                       'price' => $item->PRICE_VAT->__toString(),
                       'price_initial' => $item->PRICE_VAT_SALE->__toString(),
                       'language_id' => 1,
                       'subcategory_id' => 0,
                       'head' => null,
                       'ean' => null,
                       'code' => null,
                       'category_id' => 0,
                       'subcategory_id' => 0,
                       'action' => 0,
                       'rank' => 0,
                       'keywords' => "",
                       'published' => 0];

        }
        return $data;
    }

But when I try to use the values in this load_existing_items function I get warning undefined array key:

function load_existing_items(){
        global $db;

        $data = $this->load_xml();

        foreach($data as $key => $value){
            $item_url = $data['item_url'];
            $merchant = $data['merchant'];
            $result = $db->query("SELECT url FROM item WHERE  url = '$item_url' AND manufacturer = '$merchant'");

            while(($row = mysqli_fetch_assoc($result))) {
            $this->$db_items[] = $row['url'];
        }
        }
    }   

This is most likely some kind of a really primitive mistake that I made last night while I was desperately finish the code. Thank you for any advice/help.