Explain the Pipeline pattern in Laravel and how middleware executes using Onion Architecture.

Onion Architecture in Laravel

In Laravel, HTTP requests pass through middleware like layers of an onion. Middleware executes in sequence going inward, reaches the core route handler, and returns outward in reverse order.

Using Pipeline for Custom Business Logic

$sanitizedOrder = app(Pipeline::class)
    ->send($rawPayload)
    ->through([
        StripHtmlTags::class,
        NormalizePhoneNumbers::class,
        ApplyTaxExemptions::class,
        ValidatePromoCodes::class,
    ])
    ->then(fn ($payload) => Order::create($payload));

Each pipe implements a handle($passable, Closure $next) method:

class NormalizePhoneNumbers
{
    public function handle($payload, Closure $next)
    {
        $payload['phone'] = preg_replace('/[^0-9+]/', '', $payload['phone']);
        return $next($payload);
    }
}