Horde 6 needs the Composer Package Manager.
Currently, composer 2.2 - 2.4 are supported.
Install only for you as a normal user
<code>phive install composer</code>
OR
Install for all users as root into /usr/local/bin
<code>phive install composer</code>
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
<code>sudo zypper in php-composer2</code>
<code>sudo apt install composer</code>
See Phar.io Phive or manual download