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?

A middleware pipeline chains together multiple middleware functions, with each middleware receiving both the current request and a reference to the next piece of code in the chain, and by explicitly calling that next function, a middleware allows the request to continue on to the following middleware or eventually the actual route handler, while a middleware can also choose not to call next at all, effectively short circuiting the pipeline and immediately returning its own response, such as rejecting an unauthenticated request before it ever reaches the intended route.

function logMiddleware($request, $next) {
    error_log('Incoming request: ' . $request->path);
    $response = $next($request);
    error_log('Response sent');
    return $response;
}