Pantheon has updated the default PHP version for the Drupal (Composer Managed) upstream to PHP 8.3, replacing the previous default of PHP 8.2.
New Drupal sites created from Pantheon's upstreams use PHP 8.3. Existing sites that do not set php_version in their own pantheon.yml move to PHP 8.3 when they apply this upstream update. When you apply the update, Pantheon also updates config.platform.php in your composer.json to match, if it does not already.
Sites started from the Drupal 11 start state will see this update, but should already be running PHP 8.3.
Action required
Test the update before you deploy it to the Live environment. Apply it in Dev or a Multidev environment, confirm your modules and theme work on PHP 8.3, then promote it.
If your Drupal 10 site needs to stay on PHP 8.2, pin the version by setting php_version: 8.2 in your site's pantheon.yml before you apply the update. For steps, see Manage PHP Versions.
If you maintain a custom upstream, this change is not reflected in your pantheon.upstream.yml unless you update your fork from Pantheon's upstream.
