PSR-7 PSR-15 middleware to convert subdomain to base path

SubdomainToPathMiddleware.php

<?php

declare(strict_types=1);

namespace Api\Middleware;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;
use Zend\Expressive\Helper\UrlHelper;
use Api\Exception;

class SubdomainToPathMiddleware implements MiddlewareInterface
{
    /**
     * @var string
     */
    private $baseDomain;

    /**
     * @var UrlHelper
     */
    private $urlHelper;

    /**
     * @param string    $baseDomain
     * @param UrlHelper $urlHelper
     */
    public function __construct(string $baseDomain, UrlHelper $urlHelper)
    {
        $this->baseDomain = $baseDomain;
        $this->urlHelper = $urlHelper;
    }

    /**
     * @param ServerRequestInterface  $request
     * @param RequestHandlerInterface $handler
     *
     * @return ResponseInterface
     */
    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
    {
        $uri = $request->getUri();

        $host = $uri->getHost();

        // no subdomain or host is something less than baseDomain
        if (strlen($host) <= strlen($this->baseDomain)) {
            return $handler->handle($request);
        }

        // if request host does not end in baseDomain, dump
        if (0 !== strpos(strrev($host), strrev($this->baseDomain))) {
            return $handler->handle($request);
        }

        $subdomain = substr(
            $host,
            0,
            strlen($host) - strlen($this->baseDomain)
        );

        // if $subdomain does not end in a dot, something's wrong
        if ('.' !== substr($subdomain, -1)) {
            throw Exception\RuntimeException(
                "Subdomain '$subdomain' does not end with a dot"
            );
        }

        $path = $uri->getPath();

        $newBasePath = '/' . str_replace('.', '/', $subdomain);

        // request already includes the base path
        if (0 === strpos($path, $newBasePath)) {
            return $handler->handle($request);
        }

        $newPath = $newBasePath . $path;

        $request = $request->withUri($uri->withPath($newPath));

        return $handler->handle($request);
    }
}

You'd then write your factory and pass in the UrlHelper and the base domain. Here's mine:

<?php

declare(strict_types=1);

namespace Api\Middleware;

use Psr\Container\ContainerInterface;
use Zend\Expressive\Helper\UrlHelper;

class SubdomainToPathMiddlewareFactory
{
    public function __invoke(ContainerInterface $container): SubdomainToPathMiddleware
    {
        $domain = $container->get('config')['general']['domain'];

        return new SubdomainToPathMiddleware(
            $domain,
            $container->get(UrlHelper::class)
        );
    }
}

... and load it into your pipeline near the beginning for (optionally) specific subdomains:

...
use function Zend\Stratigility\host;
use Api\Middleware\SubdomainToPathMiddleware;

/*
 * Setup middleware pipeline:
 */
return function (Application $app, MiddlewareFactory $factory, ContainerInterface $container): void {
    // The error handler should be the first (most outer) middleware to catch
    // all Exceptions.
    $app->pipe(ErrorHandler::class);

    // "rewrite" path to start with "/api" when the subdomain is "api"
    $app->pipe(
        host(
            'api.' . $container->get('config')['general']['domain'],
            $factory->lazy(SubdomainToPathMiddleware::class)
        )
    );
...