Composer & Plesk, encountering PHP version difficulties - php

Question

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".

Answer 1

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. :-)

Answer 2

You may use platform setting in composer.json to emulate PHP version:
"config": {
"platform": {
"php": "7.1"
}
},
Composer will assume that PHP version is 7.1 during installation and update, even if you're using 7.0 to actual install.
This will only affect Composer installation and update - if you want to run console commands of your app, you will need to use correct binary (same for web server). However setting this will make your Composer installs and updates more predictable, so it's worth doing it anyway.