Since 2013, PHPTemplate is no longer used in Drupal 8 core and has been replaced by the Twig theme engine.

In 2014, we enabled Twig's autoescape feature in Drupal 8 to provide a more secure foundation for themers. To take full advantage of this feature, core relies on Twig to perform the final escaping of many variables. PHPTemplate is not compatible with this approach, currently insecure, and no longer supported, so it will be removed from Drupal 8 core.

Most Drupal themes that used PHPTemplate in Drupal 7 should be updated to use Twig. Drupal core will also still support multiple theme engines, but alternate theme engines will also need to provide some means of escaping unsafe output or risk security vulnerabilities.

Issue where this change is under discussion: Remove PHPTemplate, and add test coverage for multiple theme engine support