Theme engines

Inside core/themes there is a sixth folder (besides bartik, seven, stark, classy and stable), called engines. This folder contains the theme engines. The default template engine In Drupal 8 is Twig. The default template engine from Drupal 7 however, PHPTemplate, does still live in there. It's not recommended to continue using the PHPTemplate engine. Bear in mind that Twig has a lot more to offer and is much more secure than PHPTemplate.

What is a theme engine?

A theme engine (also called template engine, template processor or template parser) is a software component that combines data with templates from themes and shows the result - the final HTML - to the user.