Drupal 8 users who are running Drupal 8 on PHP 5.5 or PHP 5.6 should begin planning to upgrade their PHP version to 7.0 or higher. Drupal 8.6 will be the final Drupal 8 version to support PHP 5, and will reach end-of-life on March 6, 2019, when Drupal 8.7.0 is released.
If 8.7.0 is released before March 6, 2019, the release number for the end-of-life will be updated accordingly, but the end-of-life date will remain the same.
For more information, see the groups.drupal.org/core announcement about this change.
Background information
PHP 5.5 has already reached official end-of-life in 2016. PHP 5.6 also stopped receiving active support from PHP maintainers in January 2017 (i.e., it no longer receives bugfixes, even for some very serious bugs that impact Drupal development) and will reach its EOL in December 2018.
Drupal 8's automated tests require the PHPUnit library, which will drop support for PHP 5.6 in February 2018. Several other third-party dependencies are also dropping PHP 5.6 support in their latest versions.
Comments
Change in date
Unless I'm very much mistaken, because the release window for Drupal 8 minor releases got shifted back by a couple of months (since this CR was published) the actual date for dropping 8.6.x support and thus PHP 5.x support will be April 3, 2019 as per https://www.drupal.org/core/release-cycle-overview as of today.
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Regards
Steven
http://www.computerminds.co.uk