Oh dear, I've found the problem thanks to you: in FreeBSD to use the Greebo release of DokuWiki two more php extensions are needed:
devel/php72-json
and
security/php72-hash
This for the php 7.2, of course, but the same apply for older versions.
I guess that they should be considered dependencies, and I'll write to the maintainer to inform him.
Thanks you very much turnermm for your help and for your patience!
Francesco