[FIXED] Composer & Plesk, encountering PHP version difficulties

Issue

Am currently attempting to get my Laravel application running behind Plesk Onyx. Now when trying to run composer install, I get the following error messages:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for doctrine/inflector v1.3.0 -> satisfiable by doctrine/inflector[v1.3.0].
    - doctrine/inflector v1.3.0 requires php ^7.1 -> your PHP version (7.0.27) does not satisfy that requirement.
  Problem 2
    - Installation request for symfony/css-selector v4.0.6 -> satisfiable by symfony/css-selector[v4.0.6].
    - symfony/css-selector v4.0.6 requires php ^7.1.3 -> your PHP version (7.0.27) does not satisfy that requirement.
  Problem 3
    - Installation request for symfony/event-dispatcher v4.0.6 -> satisfiable by symfony/event-dispatcher[v4.0.6].
    - symfony/event-dispatcher v4.0.6 requires php ^7.1.3 -> your PHP version (7.0.27) does not satisfy that requirement.
  Problem 4
    - Installation request for symfony/translation v4.0.6 -> satisfiable by symfony/translation[v4.0.6].
    - symfony/translation v4.0.6 requires php ^7.1.3 -> your PHP version (7.0.27) does not satisfy that requirement.
  Problem 5
    - Installation request for doctrine/instantiator 1.1.0 -> satisfiable by doctrine/instantiator[1.1.0].
    - doctrine/instantiator 1.1.0 requires php ^7.1 -> your PHP version (7.0.27) does not satisfy that requirement.
  Problem 6
    - doctrine/inflector v1.3.0 requires php ^7.1 -> your PHP version (7.0.27) does not satisfy that requirement.
    - laravel/framework v5.5.39 requires doctrine/inflector ~1.1 -> satisfiable by doctrine/inflector[v1.3.0].
    - Installation request for laravel/framework v5.5.39 -> satisfiable by laravel/framework[v5.5.39].

OK. I understand the errors for sure. I run Debian 9 and "PHP by OS Vendor" is 7.0.27. However, I know that PHP 7.1+ (and even 7.2) is installed on the server, because Plesk'd let me choose (for domains and vhosts) these versions. My question now is: How can I tell composer to use a certain PHP version installed on the server, and how could this work under Debian with Plesk?

Edit: I did not install Composer globally. Just https://getcomposer.org/download/ "Command-line installation".

Solution

Here's how:

Find the PHP paths used by Plesk. In my case, this is /opt/plesk/php/. Inside this folder, there are the folders according to PHP versions installed: 7.0, 7.1, 7.2. So php composer.phar install becomes /opt/plesk/php/7.2/bin/php composer.phar install. & it works. :-)

Answered By - Johnny