How to add a Path to update-alternatives --config php

I'm working on a Ubuntu 16 machine:

lsb_release -aNo LSB modules are available.Distributor ID: UbuntuDescription:    Ubuntu 16.04.6 LTSRelease:        16.04Codename:       xenial

My experience with Ubuntu is "medium".

I needed to change the path the php command refers to in alternatives.I did the following:

cd /etc/alternativesls -ls | grep php0 lrwxrwxrwx 1 root root  15 Mar  7 06:31 php -> /usr/bin/php7.0

I want to change the path /usr/bin/php7.0 to /opt/plesk/php/7.2/bin/php.

The first thing I tried is update-alternatives --config php.This gave me this:

There is 1 choice for the alternative php (providing /usr/bin/php).

  Selection    Path             Priority   Status------------------------------------------------------------  0            /usr/bin/php7.0   70        auto mode* 1            /usr/bin/php7.0   70        manual modePress <enter> to keep the current choice[*], or type selection number:

Sadly, I can't add another path by using this command.

The second thing I tried to do is this: update-alternatives --set php /opt/plesk/php/7.2/bin/php. The result of that:

update-alternatives: error: alternative /opt/plesk/php/7.2/bin/php for php not registered; not setting

The third attempt, I went to /etc/alternatives and used unlink php. Then I created a new symlink like this: ln -s /opt/plesk/php/7.2/bin/php php.This changed the path in alternatives indead. See for yourself:

root@xxtweb03:/etc/alternatives# ls -ls | grep php0 lrwxrwxrwx 1 root root  26 Apr  4 12:18 php -> /opt/plesk/php/7.2/bin/php

It has only one disadvantage. When I run this command: update-alternatives --config php then the /opt/plesk/php/7.2/bin/php is not among the selection.

This is what I get:

root@xxtweb03:/# update-alternatives --config phpThere is 1 choice for the alternative php (providing /usr/bin/php).  Selection    Path             Priority   Status------------------------------------------------------------  0            /usr/bin/php7.0   70        auto mode  1            /usr/bin/php7.0   70        manual mode

Question: how can I add /opt/plesk/php/7.2/bin/php to the Path list that is displayed when running update-alternatives --config php?