Huh. Running PHP on the client.
I've got a couple of apps here that are distributed as .zip files that include a full Python runtime to support the client-side code, and a batch file that launches uvicorn to act as a local web server (all of which is done precisely because there isn't a remote server). I guess something similar could be done to bundle a PHP runtime and a batch script to launch its internal http server. Maybe the script could then launch the user's browser and automatically direct it to the appropriate port on the local host.
But I'm not clear on what this client-side app will actually be doing, so the whole thing may be an XY-problem.