A Single-File Router with PHP’s Built-in Server

A browser asks for /about. Somewhere on the server, a piece of code has to answer. Which one? The part of a web application that turns a URL into a piece of code is called a router, and every framework has one, however large the framework. Before you use theirs, build the smallest one that could possibly work, so you can see exactly what it does.

PHP’s built-in development server

A request has to be received by a web server, and PHP has one hiding inside the php command itself. No Apache, no nginx, nothing to install. It is not made for production. For development, and for learning, it is exactly right:

$ php -S localhost:8000 router.php
[Thu Aug 20 10:00:00 2026] PHP 8.3.0 Development Server (http://localhost:8000) started

That command starts a server on port 8000 and sends every incoming request through router.php. Nothing gets matched to a file on disk automatically. Your script decides, for each request, what to send back. Total visibility, which is exactly what we want.

The router itself

Create router.php:

<?php

$uri = parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH);

$routes = [
    '/' => function (): string {
        return "Welcome to the home page.\n";
    },
    '/about' => function (): string {
        return "This is a tiny PHP application, built without a framework.\n";
    },
];

header('Content-Type: text/plain');

if (isset($routes[$uri])) {
    echo $routes[$uri]();
} else {
    http_response_code(404);
    echo "404 Not Found: {$uri}\n";
}

Read it from the top. $_SERVER['REQUEST_URI'] is where PHP puts the path the browser asked for: /, /about, whatever was typed or clicked. parse_url(..., PHP_URL_PATH) cuts off any query string (?foo=bar), so /about?ref=email and /about land on the same route.

Then $routes is an associative array, a plain lookup table: a path on the left, and on the right a closure that produces the response. The router looks up the path; if it is a key we know, it calls the closure and echoes what comes back. Otherwise it answers 404, the way any server does for a page that does not exist.

Try it, with the server still running in another terminal:

$ curl http://localhost:8000/
Welcome to the home page.

$ curl http://localhost:8000/about
This is a tiny PHP application, built without a framework.

$ curl http://localhost:8000/nonexistent
404 Not Found: /nonexistent

Now add a route of your own: a /contact key with a closure returning any text you like. Save, and ask curl for it. No restart needed, since the server runs router.php fresh on every request.

What this is (and isn’t) doing

This router knows nothing about path parameters (/users/{id}), nothing about HTTP methods (GET versus POST at the same path), nothing about middleware. Real routers add all of that. Structurally, though, they do what these fifteen lines do: inspect something about the incoming request, and dispatch to a piece of code based on it.

A router is a lookup table with a 404 at the bottom.

Fifteen lines are enough to see the mechanism. They are not enough to grow on: the moment each route needs real HTML, the closures in that array turn into a tangle. The next section gives each route a proper home.