Magento 2.3 Upgrade issue

Question

I'm trying to upgrade to Magento 2.3.1 and I'm running into a problem. I was on PHP 7.1.3 and noticed in the system requirements that it requires 7.1.3 and 7.2.X. We've upgraded the server PHP version to 7.2.16.

Running composer update after doing the other steps brings up this error:

[user@machine public_html]$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 138 installs, 163 updates, 0 removals
  - Installing magento/inventory-composer-installer (1.1.0): Loading from cache

Warning: Unsupported declare 'strict_types' in /home/site/public_html/vendor/magento/inventory-composer-installer/src/Plugin.php on line 6

Parse error: syntax error, unexpected ':', expecting ';' or '{' in /home/site/public_html/vendor/magento/inventory-composer-installer/src/Plugin.php on line 55

Running $ php -v returns this info:

[user@machine public_html]$ php -v
PHP 7.2.16 (cli) (built: Mar 25 2019 18:26:43) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies

and in my composer.json I have:

... etc ...
"config": {
    "platform": {
        "php": "7.2.16"
    },
    "use-include-path": true
},
... etc ...

I've scoured the internet and it all leads back to upgrade PHP version - which ... it is. What step am I missing?

edit

question replies

Are you following the steps outlined in the Magento DevDocs or another resource? - Did you update using the Magento script at https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/cli-upgrade.html#upgrade-cli-script?

following the steps for manual CLI upgrade.

Do you have a cache storage system like Redis or Memcached that needs to be cleared?

Nope.

Do you have any other errors if you disable the inventory-composer-installer module?

nope.

Are you upgrading php-fpm?

Not using it.

What version of Composer do you have after upgrading?

composer -V shows

Composer version 1.5.2 2017-09-11 16:59:25

Answer 1

Updates:

Does it show any other PHP versions if you run?
sudo update-alternatives --display php
If, so then try to set PHP manually or via a command:
sudo update-alternatives --set php /usr/bin/php7.2
or sudo update-alternatives --config php

How about trying to run something like?
php7.2 /path/to/composer upgrade

End updates

Here are a couple of troubleshooting questions that may help get you in the right direction or get more info to help you further:

Are you following the steps outlined in the Magento DevDocs or another resource? - Did you update using the Magento script at https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/cli-upgrade.html#upgrade-cli-script?

Do you have a cache storage system like Redis or Memcached that needs to be cleared?

Do you have any other errors if you disable the inventory-composer-installer module?

Are you upgrading php-fpm?

What version of Composer do you have after upgrading?

Good Luck, these are some of the pain points that I had while upgrading and I hope you can get it sorted out soon.