# Creating Middleware

To create middleware, write a callable capable of receiving minimally PSR-7 ServerRequest and Response
objects, and optionally a callback to call the next in the chain.  In your middleware, you can handle
as much or as little of the request as you want, including delegating to other middleware. If your
middleware accepts a third argument, `$next`, it can allow further processing or return handling to
the parent middleware by calling it.

As an example, consider the following middleware which will use an external router to map the
incoming request path to a handler; if unable to map the request, it returns processing to the next
middleware.

```php
function ($req, $res, $next) use ($router) {
    $path = $req->getUri()->getPath();

    // Route the path
    $route = $router->route($path);
    if (! $route) {
        return $next($req, $res);
    }

    $handler = $route->getHandler();
    return $handler($req, $res, $next);
}
```

Middleware written in this way can be any of the following:

- Closures (as shown above)
- Functions
- Static class methods
- PHP array callbacks (e.g., `[ $dispatcher, 'dispatch' ]`, where `$dispatcher` is a class instance)
- Invokable PHP objects (i.e., instances of classes implementing `__invoke()`)
- Objects implementing `Zend\Stratigility\MiddlewareInterface` (including
  `Zend\Stratigility\MiddlewarePipe`)

In all cases, if you wish to implement typehinting, the signature is:

```php
function (
    Psr\Http\Message\ServerRequestInterface $request,
    Psr\Http\Message\ResponseInterface $response,
    callable $next = null
) {
}
```

The implementation Stratigility offers also allows you to write specialized error handler
middleware. The signature is the same as for normal middleware, except that it expects an additional
argument prepended to the signature, `$error`.  (Alternately, you can implement
`Zend\Stratigility\ErrorMiddlewareInterface`.) The signature is:

```php
function (
    $error, // Can be any type
    Psr\Http\Message\ServerRequestInterface $request,
    Psr\Http\Message\ResponseInterface $response,
    callable $next
) {
}
```