Avoid using ifs

if.php

<?php

//instead of using ifs in foreach
foreach ($this->currentPackingList->packingBoxes() as $packingBox) {
    if (!$this->packingService->isPackingMaterial($packingBox->itemId())) {
        continue;
    }
    $price += $this->merchantPriceForItem($packingBox->itemId()) * $this->packagingInfo->itemCount($packingBox->itemId());
}

//sometimes makes sense to have a collection 
$packingMaterials = new PackingMaterialCollection($this->currentPackingList);
foreach ($packingMaterials as $packingMaterial) {
    $price += $this->merchantPriceForItem($packingBox->itemId()) * $this->packagingInfo->itemCount($packingBox->itemId());
}

//class 
final class PackingMaterialCollection implements \IteratorAggregate
{
    /**
     * @var PackingList
     */
    private $packingList;

    /**
     * @var \CallbackFilterIterator
     */
    private $iterator;

    public function __construct(PackingList $packingList)
    {
        $this->packingList = $packingList;

        $this->iterator = new \CallbackFilterIterator(
            new \ArrayIterator($this->packingList->packingBoxes()),
            function (PackingBox $packingBox) {
                return $this->isPackingMaterial($packingBox);
            }
        );
    }

    public function isPackingMaterial(PackingBox$packingBox)
    {
        $packingMaterials = [
            PackingMaterials::BOTTLE_BOX,
            PackingMaterials::GLASS_BOX,
            PackingMaterials::DISH_BOX,
            PackingMaterials::PACKING_TAPE,
            PackingMaterials::WRAPPING_TISSUE,
            PackingMaterials::STRETCH_WRAP,
            PackingMaterials::BUBBLE_WRAP,
            PackingMaterials::CLOTHES_BOX,
            PackingMaterials::SMALL_BOX,
            PackingMaterials::PROFI_BOX
        ];
        return in_array($packingBox->itemId(), $packingMaterials);
    }

    public function getIterator()
    {
        return new \IteratorIterator($this->iterator);
    }
}