PHP Namespace Mismatch — Autoloader Fails Silently

<?php
// FILE 1: vendor/slacknotify/src/Logger.php
namespace SlackNotify\Logging;  // This file lives in the SlackNotify\Logging namespace

class Logger
{
    public function log(string $message): void
    {
        // Pretend this sends a Slack message
        echo "[SlackNotify] " . $message . PHP_EOL;
    }
}


// FILE 2: vendor/monolog/src/Logger.php
namespace Monolog;  // This file lives in the Monolog namespace

class Logger
{
    public function log(string $message): void
    {
        // Pretend this writes to a log file
        echo "[Monolog] " . $message . PHP_EOL;
    }
}


// FILE 3: index.php  — Using BOTH classes with zero conflict
require 'vendor/slacknotify/src/Logger.php';
require 'vendor/monolog/src/Logger.php';

// 'use' imports the fully-qualified name so we can write short aliases
use SlackNotify\Logging\Logger as SlackLogger;  // alias to avoid ambiguity
use Monolog\Logger as MonologLogger;             // alias to avoid ambiguity

$slackLogger   = new SlackLogger();    // creates SlackNotify\Logging\Logger
$monologLogger = new MonologLogger();  // creates Monolog\Logger

$slackLogger->log('User signed up');    // routes to SlackNotify version
$monologLogger->log('User signed up');  // routes to Monolog version

<?php
declare(strict_types=1);  // declare() is the ONE thing allowed before namespace

namespace App\Services\Payments;  // mirrors the directory: src/Services/Payments/

// No imports needed yet — everything in THIS namespace is available by default

class PaymentGateway
{
    private string $providerName;
    private float  $transactionFee;

    public function __construct(string $providerName, float $transactionFee)
    {\n        $this->providerName   = $providerName;\n        $this->transactionFee = $transactionFee;\n    }

    public function charge(float $amount): string
    {
        // Calculate total including the gateway's processing fee
        $totalCharged = $amount + $this->transactionFee;

        return sprintf(
            'Charged $%.2f via %s (includes $%.2f fee)',
            $totalCharged,
            $this->providerName,
            $this->transactionFee
        );
    }
}


// ── index.php ─────────────────────────────────────────────────────────────────
// In a real project Composer autoloads this. Here we require manually.
require 'PaymentGateway.php';

use App\Services\Payments\PaymentGateway;  // import the fully-qualified name

$stripeGateway = new PaymentGateway('Stripe', 0.30);  // $0.30 flat fee
$paypalGateway = new PaymentGateway('PayPal', 0.49);  // $0.49 flat fee

echo $stripeGateway->charge(19.99) . PHP_EOL;
echo $paypalGateway->charge(19.99) . PHP_EOL;

<?php
declare(strict_types=1);

namespace App\Http\Controllers;

// Import a third-party class with a long namespace — alias it for brevity
use App\Services\Payments\PaymentGateway;
use App\Services\Payments\PaymentGateway as Gateway;  // alias: type 'Gateway' instead

// Import global PHP classes explicitly — don't rely on implicit fallback
use DateTime;      // tells PHP: DateTime means \DateTime, the global built-in
use RuntimeException;  // same for exceptions

class OrderProcessor
{\n    private Gateway $paymentGateway;  // using the alias 'Gateway' — cleaner!\n\n    public function __construct(Gateway $paymentGateway)\n    {\n        $this->paymentGateway = $paymentGateway;\n    }

    public function processOrder(string $orderId, float $total): array
    {\n        if ($total <= 0) {\n            // 'RuntimeException' resolves to global \\RuntimeException because of our 'use' above\n            throw new RuntimeException(\"Order {$orderId} has an invalid total: {$total}\");\n        }\n\n        $chargeResult = $this->paymentGateway->charge($total);\n\n        // 'DateTime' resolves to global \\DateTime because of our 'use' above\n        $processedAt = new DateTime();\n\n        return [\n            'order_id'     => $orderId,\n            'result'       => $chargeResult,\n            'processed_at' => $processedAt->format('Y-m-d H:i:s'),\n        ];\n    }\n}\n\n\n// ── run_order.php ─────────────────────────────────────────────────────────────\nrequire 'PaymentGateway.php';\nrequire 'OrderProcessor.php';\n\nuse App\\Http\\Controllers\\OrderProcessor;\nuse App\\Services\\Payments\\PaymentGateway;\n\n$gateway   = new PaymentGateway('Stripe', 0.30);\n$processor = new OrderProcessor($gateway);\n\n$result = $processor->processOrder('ORD-7821', 49.99);\n\nforeach ($result as $key => $value) {\n    echo $key . ': ' . $value . PHP_EOL;\n}",
        "output": "order_id: ORD-7821\nresult: Charged $50.29 via Stripe (includes $0.30 fee)\nprocessed_at: 2024-03-15 14:22:07"
      }

<?php
/**
 * SIMULATED DIRECTORY STRUCTURE (in a real project, Composer handles the requires):
 *
 * src/
 *   Models/Product.php          -> namespace App\Models
 *   Repositories/ProductRepo.php -> namespace App\Repositories
 *   Controllers/ShopController.php -> namespace App\Controllers
 * index.php
 *
 * composer.json autoload section:
 * "autoload": { \"psr-4\": { \"App\\\\\": \"src/\" } }\n */\n\n// ── src/Models/Product.php ────────────────────────────────────────────────────\nnamespace App\\Models;\n\nclass Product\n{\n    public function __construct(\n        public readonly int    $id,\n        public readonly string $name,\n        public readonly float  $price,\n        public readonly int    $stockQuantity\n    ) {}\n\n    public function isInStock(): bool\n    {\n        return $this->stockQuantity > 0;  // simple stock check\n    }\n\n    public function getFormattedPrice(): string\n    {\n        return '$' . number_format($this->price, 2);  // format to 2 decimal places\n    }\n}\n\n\n// ── src/Repositories/ProductRepository.php ───────────────────────────────────\nnamespace App\\Repositories;\n\nuse App\\Models\\Product;  // import Product from the Models namespace\n\nclass ProductRepository\n{\n    /** @var Product[] */\n    private array $products = [];\n\n    public function __construct()\n    {\n        // Seed with demo data — in production this would query a database\n        $this->products = [\n            new Product(1, 'Mechanical Keyboard', 129.99, 14),\n            new Product(2, 'USB-C Hub',            39.99,  0),  // out of stock\n            new Product(3, 'Monitor Stand',         59.99,  7),\n        ];\n    }\n\n    /** @return Product[] */\n    public function findAllInStock(): array\n    {\n        // Filter to only products with stock > 0\n        return array_filter($this->products, fn(Product $p) => $p->isInStock());\n    }\n\n    public function findById(int $productId): ?Product\n    {\n        foreach ($this->products as $product) {\n            if ($product->id === $productId) {\n                return $product;  // found it — return immediately\n            }\n        }\n        return null;  // not found — return null, let the caller decide what to do\n    }\n}\n\n\n// ── src/Controllers/ShopController.php ───────────────────────────────────────\nnamespace App\\Controllers;\n\nuse App\\Repositories\\ProductRepository;  // import the repository\nuse RuntimeException;                    // global PHP exception — explicit import\n\nclass ShopController\n{\n    private ProductRepository $productRepository;\n\n    public function __construct(ProductRepository $productRepository)\n    {\n        // Dependency injected — no 'new ProductRepository()' buried in here\n        $this->productRepository = $productRepository;\n    }\n\n    public function listInStockProducts(): void\n    {\n        $inStockProducts = $this->productRepository->findAllInStock();\n\n        echo \"=== In-Stock Products ==\" . PHP_EOL;\n        foreach ($inStockProducts as $product) {\n            echo sprintf(\n                \"  [%d] %-22s %s (%d in stock)%s\",\n                $product->id,\n                $product->name,\n                $product->getFormattedPrice(),\n                $product->stockQuantity,\n                PHP_EOL\n            );\n        }\n    }\n\n    public function showProduct(int $productId): void\n    {\n        $product = $this->productRepository->findById($productId);\n\n        if ($product === null) {\n            throw new RuntimeException(\"Product ID {$productId} not found.\");\n        }\n\n        $stockStatus = $product->isInStock() ? 'In Stock' : 'OUT OF STOCK';\n\n        echo PHP_EOL . \"=== Product Detail ==\" . PHP_EOL;\n        echo \"Name:   {$product->name}\" . PHP_EOL;\n        echo \"Price:  {$product->getFormattedPrice()}\" . PHP_EOL;\n        echo \"Status: {$stockStatus}\" . PHP_EOL;\n    }\n}\n\n\n// ── index.php (entry point) ───────────────────────────────────────────────────\n// In a real project: require 'vendor/autoload.php'; — that's the only require needed\nuse App\\Repositories\\ProductRepository;\nuse App\\Controllers\\ShopController;\n\n$repository = new ProductRepository();\n$controller = new ShopController($repository);  // inject the dependency\n\n$controller->listInStockProducts();  // list everything in stock\n$controller->showProduct(2);         // show USB-C Hub — which is out of stock",
        "output": "=== In-Stock Products ==\n  [1] Mechanical Keyboard    $129.99 (14 in stock)\n  [3] Monitor Stand          $59.99 (7 in stock)\n\n=== Product Detail ==\nName:   USB-C Hub\nPrice:  $39.99\nStatus: OUT OF STOCK"
      }

<?php
namespace App\Utils;

// No import for json_encode
// In PHP >=8.0, this will throw "Call to undefined function App\Utils\json_encode"
// In PHP <8.0, it might work if no local function shadows it

echo json_encode(['test' => 'value']);  // Works in <8.0, fails in >=8.0

// The correct approach:
use function json_encode;  // import the global function explicitly
echo json_encode(['test' => 'value']);  // Works in all versions

// Or for a single use, prefix with backslash:
echo \json_encode(['test' => 'value']);  // Always works

// Constants example:
// Inside a namespace, PHP_EOL refers to App\Utils\PHP_EOL if defined, else global?
// Actually constants follow similar rules - explicit import is safest.
use const PHP_EOL;
echo 'Line ends with' . PHP_EOL;