CompanyIdentifierResolver.php

<?php

declare(strict_types=1);

namespace Dafis\Application\Middleware;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\RequestHandlerInterface;
use TYPO3\CMS\Core\Utility\MathUtility;

class CompanyIdentifierResolver implements MiddlewareInterface
{
    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
    {
        $path = ltrim($request->getUri()->getPath(), '/');
        if (empty($path)) {
            return $handler->handle($request);
        }

        $pathParts = explode('/', $path);
        $companyCandidate = $pathParts[0];

        if (MathUtility::canBeInterpretedAsInteger($companyCandidate)) {
            $request = $request->withAttribute('company_id', (int) $companyCandidate);

            $uri = $request->getUri();
            array_shift($pathParts);
            $uri = $uri->withPath('/' . implode('/', $pathParts));

            $request = $request->withUri($uri);
        }

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

RequestMiddlewares.php

<?php

return [
    'frontend' => [
        'dafis/application/company-identifier-resolver' => [
            'target' => \Dafis\Application\Middleware\CompanyIdentifierResolver::class,
            'before' => [
                'typo3/cms-frontend/site'
            ]
        ]
    ]
];