The theme engine PHPTemplate has been removed from Drupal, because the lack of support for autoescaping made it fundamentally insecure. Drupal will continue to support alternative theme engines, but only Twig will be included with core.
If you use an alternative theme engine, you will need to provide some means of escaping unsafe output or security vulnerabilities will result. This is handled in Drupal core by Twig's autoescape.
Let the Nyan Cat guide you on your way to Twig happiness.
Related change records
See Twig autoescape enabled and text sanitization APIs updated for a full list of related change records.