Purpose: Modify method arguments before the original method executes.

Signature: public function before<MethodName>($subject, $argument1, ...)

Return: Array of modified arguments (or null to keep original)

Example: Add customer group discount to product price calculation

<?php
declare(strict_types=1);

namespace Vendor\Module\Plugin\Catalog\Model\Product;

use Magento\Catalog\Model\Product;
use Magento\Customer\Model\Session as CustomerSession;
use Psr\Log\LoggerInterface;

class PriceExtend
{
    public function __construct(
        private readonly CustomerSession $customerSession,
        private readonly LoggerInterface $logger
    ) {
    }

    public function beforeGetFinalPrice(Product $subject, $qty = 1.0): ?array
    {
        $customerGroupId = $this->customerSession->getCustomerGroupId();

        // Example: Apply bulk discount for wholesale customer group (ID 3)
        if ($customerGroupId === 3 && $qty < 10) {
            $this->logger->info('Adjusting quantity to 10 for wholesale pricing');
            return [10.0]; // Force minimum quantity for wholesale price tier
        }

        return null; // Return null to keep original arguments unchanged
    }
}

Purpose: Modify the return value after the original method executes.

Signature: public function after<MethodName>($subject, $result, $argument1, ...)

Return: Modified result (must match original return type)

Example: Add custom attribute to product collection

<?php
declare(strict_types=1);

namespace Vendor\Module\Plugin\Catalog\Model\ResourceModel\Product;

use Magento\Catalog\Model\ResourceModel\Product\Collection;
use Psr\Log\LoggerInterface;

class CollectionExtend
{
    public function __construct(
        private readonly LoggerInterface $logger
    ) {
    }

    public function afterLoad(Collection $subject, Collection $result): Collection
    {
        if ($result->count() === 0) {
            return $result;
        }

        foreach ($result->getItems() as $product) {
            $customValue = $this->calculateCustomValue($product);
            $product->setData('custom_attribute', $customValue);
        }

        $this->logger->debug('Added custom attribute to product collection', [
            'product_count' => $result->count()
        ]);

        return $result;
    }

    private function calculateCustomValue($product): string
    {
        return 'custom_' . $product->getId();
    }
}

Purpose: Completely control method execution (call original, skip it, or replace it).

Signature: public function around<MethodName>($subject, callable $proceed, $argument1, ...)

Use Case: Conditional execution, caching, performance optimization

Example: Add caching layer to expensive product recommendation

<?php
declare(strict_types=1);

namespace Vendor\Module\Plugin\Catalog\Model\Product;

use Magento\Framework\App\CacheInterface;
use Magento\Framework\Serialize\SerializerInterface;

class RecommendationExtend
{
    private const CACHE_KEY_PREFIX = 'product_recommendations_';
    private const CACHE_LIFETIME = 3600;
    private const CACHE_TAG = 'product_recommendations';

    public function __construct(
        private readonly CacheInterface $cache,
        private readonly SerializerInterface $serializer
    ) {
    }

    public function aroundGetRecommendations(
        $subject,
        callable $proceed,
        $product,
        int $limit = 5
    ): array {
        $cacheKey = self::CACHE_KEY_PREFIX . $product->getId() . '_' . $limit;

        // Try cache first
        $cachedData = $this->cache->load($cacheKey);
        if ($cachedData !== false) {
            return $this->serializer->unserialize($cachedData);
        }

        // Cache miss - call original method
        $result = $proceed($product, $limit);

        // Store in cache
        $this->cache->save(
            $this->serializer->serialize($result),
            $cacheKey,
            [self::CACHE_TAG],
            self::CACHE_LIFETIME
        );

        return $result;
    }
}