PHP Template
Understand why Twig is replacing PHP Template in Drupal
A completely new template engine is introduced in Drupal. The templating system in all previous versions of Drupal is using theme_* functions and PHP-based *.tpl.php files. Now, this has been completely replaced in Drupal 8. Instead of using theme functions and theme template files, Drupal is now using the Twig templating system.
Twig is a PHP-based compiled templating