Magento 2 : Error While upgrade 2.3.0 to 2.3.1

Question

I am trying to upgrade Magento version from 2.3.0 to 2.3.1 and I am using PHP 7.2 for same but while I run upgrade command getting bellow error is anyone have any idea how to fix?

 Parse error: syntax error, unexpected ':', expecting ';' or '{' in /vendor/magento/inventory-composer-installer/src/Plugin.php on line 55
 Warning: Unsupported declare 'strict_types' in /vendor/magento/inventory-composer-installer/src/Plugin.php on line 6
 Parse error: syntax error, unexpected ':', expecting ';' or '{' in /vendor/magento/inventory-composer-installer/src/Plugin.php on line 55

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• Raj Mohan R: Magento 2.3.1 is compatible with 7.2.x only
• Navin Bhudiya: i also try to update at 7.2.x still also same error
• Muhammad Hasham: Magento 2.3.1 is compatible with PHP 7.2.x and one more thing have you checked your php version in command interface using php -v ?
• Raj Mohan R: Can you please share your console screenshot?
• Navin Bhudiya: added, please check

Answer 1

Magento does not require PHP 7.2 since 2.3.1 release, PHP 7.1.3 and later still supported. https://devdocs.magento.com/guides/v2.3/install-gde/system-requirements-tech.html

in your case, the system has a fatal error encountering void return type https://github.com/magento-engcom/inventory-composer-installer/blob/master/src/Plugin.php#L55

Which is added since PHP 7.1 release - https://www.php.net/manual/en/migration71.new-features.php

Answer 2

it was due to a server issue, when I run composer update command it taking wrong PHP version due to some server configuration. so I just download composer.phar file for run composer update using PHP version and it worked. for me, bellow command working php composer.phar update

Magento 2.3.1 required php 7.1 version if version is less then php 7.1 then it give error in my case default php version was 5.6 something so it giving error