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What is routing in a PHP web application, and how does a router determine which piece of code should handle a specific incoming request? How does a middleware pipeline execute multiple middleware in sequence, and how does the concept of calling the next middleware let each one decide whether to continue processing the request? How do route parameters and route constraints let you capture dynamic segments of a URL while also restricting what values are actually considered a valid match? How can route groups let you apply shared configuration, such as a common URL prefix or a set of middleware, to several related routes at once? How does middleware priority and ordering affect application behavior when multiple middleware, such as authentication and rate limiting, are applied to the same route together? How can a custom routing system be designed to support efficient route matching performance even as an application grows to have hundreds or thousands of defined routes?
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What is middleware in the context of a web application, and how does it let you run logic before or after a request is handled by its intended route?

Beginner

Answer

Middleware is a piece of code that sits within the request handling pipeline, running either before a request reaches its intended route handler, after the handler produces a response, or both, letting you implement cross cutting concerns, such as authentication checks, logging, or modifying request and response data, in a reusable way that can be applied consistently across many different routes without duplicating that same logic within every individual route handler.
function authMiddleware($request, $next) {
    if (!$request->user) {
        return redirect('/login');
    }
    return $next($request);
}
Real-world example An application applies an authentication middleware to every route within its admin panel, ensuring the exact same login check runs consistently before any admin route is ever reached, without needing to repeat that check inside every single admin controller method.

Common follow-ups: What is the difference between global middleware and route specific middleware?;How does middleware relate to the concept of the request response lifecycle?

MVC Architecture in PHP;Security
All 7 Routing & Middleware questions