Composer does not find my php 7.2 on server.

I have an Debian 8 on server but and has got added PHP 7.2.
I have set up a site and choosed php7.2 as php version.
I have installed composer and got no error.
I log in as root on server adn move to the directory that I want the app to be installed on and I get several error swhen I run a composer command for installing and app, it is all related to php version, it states that php 7 or newver is not avaliable.
How can I fix this?

Example error :
- phpunit/phpunit 6.5.9 requires php ^7.0 -> your PHP version (5.6.40) does not satisfy that requirement.

 

use

Code:

sudo -u webXY /usr/bin/php7.2 YourCommand