Any computer program can be designed to run from a single file if you architect it wrong enough! I wanted to create the simplest possible Fediverse server which can be used as an educational tool to show how ActivityPub / Mastodon works. The design goals were: Upload a single PHP file to the server. No [...]
Seems like decent example code.
Technically any PHP site could be in a single file. You can even embed images if you base64 encode them.
I guess what they mean is a single file with a reasonable size.