OK, there’s one ugly (sorry) workaround for the case when you have one output format… output both HTML and PHP, but configure your server to look for index.php before index.html.
site configuration
[outputFormats.PHP]
mediaType = "application/x-php"
rel = "canonical"
isHTML = true
permalinkable = false # IMPORTANT
front matter
+++
title = 'Test'
date = 2021-01-01T00:00:00-00:00
draft = false
outputs = ['HTML','PHP']
+++