Hallo,
I use plesk version 17.8.11. I configured php-version 7.2.4, but a php -i in the bash shows version 7.0.28:
$ php -i
phpinfo()
PHP Version => 7.0.28-0ubuntu0.16.04.1
A software, which needs at least php version 7.1 shows with composer in the bash, that I'm using version 7.0.28, too...