The PHPTemplate will no longer be used in Drupal 8 core. How is that even possible? Simple, by finding a replacement, and no replacement was better than the Twig theme engine.
Twig's auto-escape feature was enabled in Drupal 8 for an increased level of security. With Drupal 8, your themes will have a more secure foundation. To enable this feature, Twig had to do the final escaping of many variables. As PHPTemplate is no longer compatible, it will be removed from Drupal 8 core.
Attention themers: most Drupal themes with PHPTemplate in Drupal 7 must be updated to use Twig. The best part is that Drupal core will still support multiple theme engines, on the condition that alternate theme engines provide at least one means of protecting the system against unsafe output or risk security vulnerabilities.