I just upgraded PHP from 7.3 to 7.4. I have done testing locally and on a staging site, and since it's a minor version update and PHPBB3 officially supports 7.4 I don't anticipate any issues. But if you notice any please report them here or contact me directly, thank you!
I'm planning to go to 8.0 soon and then 8.1, since according to phpbb docs they do fully support 8.1 and there's no reason (AFAIK) that it would cause any problems for the main site.
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