For updating, you are still technically running 2.2 so you will need to use php7.1 after you have updated to 2.3 then you can switch to php 7.2.
Also the upgrade can only be done from 2.2.6 -> 2.3.0
Upgrading from Magento 2.2.6 to 2.3.0 isn't as clear cut as it has been in the past. Please follow the directions in the devdocs to do a proper upgrade to 2.3.0
https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/cli-upgrade.html#upgrade-cli-script
Which is condensed below:
1) composer require magento/product-community-edition 2.3.0 --no-update
2) composer config preferred-install dist
3) composer config sort-packages true
4) composer config prefer-stable true
5) composer require --dev friendsofphp/php-cs-fixer:~2.10.0 --no-update
6) composer require --dev sebastian/phpcpd:~3.0.0 --no-update
7) php -r '$autoload=json_decode(file_get_contents("composer.json"), true); $autoload["autoload"]["psr-4"]["Zend\Mvc\Controller\"]= "setup/src/Zend/Mvc/Controller/"; file_put_contents("composer.json", json_encode($autoload, JSON_PRETTY_PRINT|JSON_UNESCAPED_SLASHES));'
(copy this as one line, please)
8) composer update
9) php bin/magento setup:upgrade
10) php bin/magento setup:static-content:deploy
11) php bin/magento setup:di:compile
12) php bin/magento cache:flush
Although you probably should keep php at 7.1 until you actually finish the upgrade since you are technically still on Magento 2.2.6 which does require php 7.1 only switch to php 7.2 after upgrade is complete.
Best Answer
To resolve I manually updated the packages that gave me error one by one. Every update I did rerun
composer:update. In the end all the requirements were met and the update was successful. The PHP version was correct.Now for the upgrade to version 2.3.2 I found the same problem. I found incompatible packages in composer.json and removed them. Then I discovered the existence of this parameter that was useful to me:
composer require magento/product-community-edition 2.3.2 --update-with-all-dependencies.