It's a niche format but PrismJS has a number of fairly niche formats already so maybe one more wouldn't be a big issue.

Language
Quoting https://phpseclib.com/docs/phpbb :

If you're going to be applying code changes by hand the various diff formats are of limited usefulness. They're great for automated tools like patch but not so much for by hand edits.

A more useful format for by hand edits is the phpBB MOD Text Template. Unfortunately, as of April 3, 2022 the phpBB wiki wherein the format was described is no more, hence this page, which has been adapted from archive.org.

Additional resources
https://phpseclib.com/docs/phpbb