SOLID & Design Principles - OOP Fundamentals

What is Encapsulation?

Encapsulation is the bundling of data (properties) and methods that operate on that data into a single unit (class), while restricting direct access to some components. This protects object integrity by preventing external code from putting the object into an invalid state.

Access Modifiers in PHP

PHP provides three access modifiers:

Example: Encapsulated Product Price

namespace Vendor\Catalog\Model;

class PriceManager
{
    private float $basePrice;
    private float $taxRate;
    private float $discountPercent = 0;

    public function __construct(float $basePrice, float $taxRate)
    {
        $this->setBasePrice($basePrice);
        $this->taxRate = $taxRate;
    }

    private function setBasePrice(float $price): void
    {
        if ($price < 0) {
            throw new \InvalidArgumentException('Price cannot be negative');
        }
        $this->basePrice = $price;
    }

    public function applyDiscount(float $percent): void
    {
        if ($percent < 0 || $percent > 100) {
            throw new \InvalidArgumentException('Discount must be 0-100');
        }
        $this->discountPercent = $percent;
    }

    public function getFinalPrice(): float
    {
        $discounted = $this->basePrice * (1 - $this->discountPercent / 100);
        return $discounted * (1 + $this->taxRate);
    }

    public function getBasePrice(): float
    {
        return $this->basePrice;
    }
}

Why This Matters

Without encapsulation, external code could set $basePrice = -100 and break calculations. The validated setter prevents invalid states. In Magento, nearly every model uses encapsulation — look at Magento\Catalog\Model\Product which has private/protected properties with controlled access via getters and setters.

Magento Example: Product Entity

// Magento\Catalog\Model\Product (simplified)
class Product extends \Magento\Framework\Model\AbstractModel
{
    protected $sku;
    protected $name;
    protected $price;

    public function getSku(): ?string
    {
        return $this->getData('sku');
    }

    public function setSku(string $sku): ProductInterface
    {
        return $this->setData('sku', $sku);
    }
}

The setData method provides a controlled way to modify internal state, maintaining encapsulation even with the generic data array pattern.