PHP 5.5 only supported for existing Drupal 8 sites, new Drupal 8 sites require PHP 7.0.8

PHP stopped supporting version 5.5 on July 21, 2016 (two years and eight months ago) and version 5.6 on December 2018. Drupal's end of support for PHP 5.5 and 5.6 was announced in January 2018.

New sites require at least PHP 7.0.8

New sites can only be installed on PHP 7.0.8 or later. Installing Drupal on older versions results in a requirement error.

The new DRUPAL_MINIMUM_SUPPORTED_PHP constant was added to support this. New sites cannot be installed on PHP versions older than this version.

Existing sites still work on at least PHP 5.5.9 for now, but will display a warning

The DRUPAL_MINIMUM_PHP constant's meaning changed. Existing sites running at least on PHP 5.5.9 can still run Drupal updates, but if the version is older than DRUPAL_MINIMUM_SUPPORTED_PHP (7.0.8 at this time) a warning is displayed.

Update to PHP 7.0.8 or higher before December 2019 to continue receiving Drupal security updates

Note that Drupal security updates will begin requiring PHP 7 as early as Drupal 8.8.0 (December 2019), so all users are advised to update to at least PHP 7.0.8 now.

Test supporting code changes

Drupal\Tests\RequirementsPageTrait was added to help testing the update and install requirements pages.

Use the Drupal\Tests\RequirementsPageTrait::updateRequirementsProblem function in tests to skip the PHP version warning when on the update requirements page.

Drupal\simpletest\InstallerTestBase::continueOnExpectedWarnings, Drupal\FunctionalTests\Installer\InstallerTestBase::continueOnExpectedWarnings methods are no longer called by any core tests. They were refactored and moved to Drupal\Tests\RequirementsPageTrait.