Horde 6 needs the Composer Package Manager.
Currently, composer 2.2 - 2.4 are supported.
Install only for you as a normal user
phive install composer
OR
Install for all users as root into /usr/local/bin
phive install composer
See https://getcomposer.org/download/ for instructions. Place the downloaded file in /usr/local/bin or another location in the search path and set the executable bit
sudo zypper in php-composer2
sudo apt install composer
See Phar.io Phive or manual download
See Mailing list:
https://marc.info/?l=horde&m=167152123830875&w=2
sudo apt install composer