Building A PHP Routing System

What is Routing?

Routing is the process of parsing a URI and determining the appropriate action to take.

For example, consider the following URI:

http://example.com/login

How does a request like the one above result in a response to the user?

Let’s break it down:

1. The request is received by the application.
2. The application breaks down the request into its components. Things like: the method (ex: GET), host, path, etc.
3. The application looks for a defined route that matches this request.
4. Once found, it takes the defined action and returns a response.

Example from Laravel

For a real-world example, here’s one way the above example could be implemented in Laravel.

Route::get('/login', function() {
    return view('login');
});

We define a GET route for the /login URI.

When that route is requested we return a response – in this case, HTML for the login page.

How Routers Work

There are a number of routers in the PHP ecosystem.

They range from the more simplistic to feature-packed behemoths.

Although they differ in size and complexity, they generally employ the same fundamental steps: parse the request, match the pattern, run some code, return a response.

The Symfony router (probably the most widely used PHP router) does this.

As does this one.

And this one.

A Very Simple PHP Router

To demonstrate these concepts let’s create a stupid simple, not at all useful, PHP router.

<?php
/**
 * First, let's define our Router object.
 */
class Router
{
    /**
     * The request we're working with.
     *
     * @var string
     */
    public $request;

    /**
     * The $routes array will contain our URI's and callbacks.
     * @var array
     */
    public $routes = [];

    /**
     * For this example, the constructor will be responsible
     * for parsing the request.
     *
     * @param array $request
     */
    public function __construct(array $request)
    {
        /**
         * This is a very (VERY) simple example of parsing
         * the request. We use the $_SERVER superglobal to
         * grab the URI.
         */
        $this->request = basename($request['REQUEST_URI']);
    }

    /**
     * Add a route and callback to our $routes array.
     *
     * @param string $uri
     * @param Callable $fn
     */
    public function addRoute(string $uri, \Closure $fn) : void
    {
        $this->routes[$uri] = $fn;
    }

    /**
     * Determine is the requested route exists in our
     * routes array.
     *
     * @param string $uri
     * @return boolean
     */
    public function hasRoute(string $uri) : bool
    {
        return array_key_exists($uri, $this->routes);
    }

    /**
     * Run the router.
     *
     * @return mixed
     */
    public function run()
    {
        if($this->hasRoute($this->request)) {
            $this->routes[$this->request]->call($this);
        }
    }
}

/**
 * Create a new router instance.
 */
$router = new Router($_SERVER);

/**
 * Add a "hello" route that prints to the screen.
 */
$router->addRoute('hello', function() {
    echo 'Well, hello there!!';
});

/**
 * Run it!
 */
$router->run();

Run The Code

1. Save this code locally as index.php.
2. In your terminal navigate to the directory where you saved the script.
3. Start the built-in PHP web server: php -S localhost:1234
4. In your browser go to:

http://localhost:1234/hello

Conclusion

I’ve touched on the very basics of routing, shared some routing examples from the PHP world, and built a extremely simple router.

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