While updating from 3.0.14 to 3.1.9 and following the upgrade guide https://www.phpbb.com/support/docs/en/3.1/ug/upgradeguide/upgrade3/, I started the DB upgrade from the CLI:
php ./bin/phpbbcli.php db:migrate --safe-mode
After some success lines I encountered a fatal error:
This is solved by adding
require($phpbb_root_path . 'includes/functions_content.' . $phpEx);
This is a different issue from https://tracker.phpbb.com/browse/PHPBB3-14372, as it affects core installation / update, an not an extension.