Hi @Jan Vávra ,
You can run 7.3 and 7.4 runtimes on Windows hosted app service plans; however, support for those runtimes is scheduled to end December 2021 and November 2022, respectively. PHP 8 will only be offered for Linux hosted app service plans. See https://github.com/Azure/app-service-linux-docs/blob/master/Runtime_Support/php_support.md#php-8 for more information.
I understand you have a C++ library that you're interested in running. Therefore, I would suggest containerizing your application to a Windows Server core LTS base image, configuring PHP runtime, and running your app code inside the container if you're set on using that extension. Please refer to this how-to guide on how to get started with running custom containers on Azure.