Silvestrus both 8.2 and 8.3 work ok as long as you disable deprecation warnings. 8.3 seems to be the current stable and Flarum should work with it. 8.4 might be too risky.
Make sure to run composer update --preer-dist --no-dev -a whenever you change php versions, this will make sure that all dependencies compatible with the respective hosting environment are loaded. Understand this also updates all extensions.
