TWIG in Drupal 8: getting started
TWIG is in core for Drupal 8. However the core themes (bartik, seven, stark) aren't converted yet, and you won't find any .twig templates in the current 8.x-dev. The theme conversion is taking place in a seperate Drupal 8 Twig sandbox. A great deal of explanation is done on that project page and under the "Get started" section. Basically, all these issues have to be done. Also, for more background info, check out the video about Twig in DrupalCon Sydney. Will phptemplate still exist in Drupal 8? For now it seems so (correct me if I'm wrong), but I guess that might change until code freeze based on the amount of progress.