install dependencies via Composer (small note: ext-yaml is used only for configuration parsing, so if you want to drop this dependency, simply adjust ./application/container.php)
create cache folder: mkdir -p ./data/cache/ZendCache (you will need it later for configuration and metadata caches - check comments in ./application/config.yml)
start PHP development server: php -S 0.0.0.0:8080 -t ./public (or any other SAPI you prefer, just make sure that you configure webroot to ./public and rewrites to ./public/index.php)

After that you should be able to call all methods from your API spec. Most of the negative scenarios should be handled:

404 Not found for unknown routes
406 Not acceptable for invalid Accept header
415 Unsupported media type for invalid Content-Type header
400 Malformed JSON for unparsable JSON body
422 Unprocessable entity for parsable JSON body that fails validation

But for obvious reason you will not get any 200 OK, only 501 Not implemented. So your next steps are:

check all TODOs left in the stub code where generator was not smart enough and could not guarantee correct implementation
implement your API security mechanism (either special attribute or separate middleware) - generator does not do anything about it yet
implement your handlers - the most tricky part :)

Enjoy!

Hopefully this stub will reduce the amount of boilerplate code you have to write manually. If you have any suggestions or questions about php-ze-ph generator, feel free to create issue either in Path Handler repository or in OpenAPI Generator repository.