Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/themes/engines/phptemplate/phptemplate.engine

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+<?php
+
+/**
+ * @file
+ * Handles integration of PHP templates with the Drupal theme system.
+ */
+
+use Drupal\Core\Extension\Extension;
+
+/**
+ * Implements hook_init().
+ */
+function phptemplate_init(Extension $theme) {
+  $theme->load();
+}
+
+/**
+ * Implements hook_theme().
+ */
+function phptemplate_theme($existing, $type, $theme, $path) {
+  $templates = drupal_find_theme_functions($existing, array($theme));
+  $templates += drupal_find_theme_templates($existing, '.tpl.php', $path);
+  return $templates;
+}
+
+/**
+ * Implements hook_extension().
+ */
+function phptemplate_extension() {
+  return '.tpl.php';
+}
+
+/**
+ * Implements hook_render_template().
+ *
+ * Renders a system default template, which is essentially a PHP template.
+ *
+ * @param $template_file
+ *   The filename of the template to render.
+ * @param $variables
+ *   A keyed array of variables that will appear in the output.
+ *
+ * @return
+ *   The output generated by the template.
+ */
+function phptemplate_render_template($template_file, $variables) {
+  // Extract the variables to a local namespace
+  extract($variables, EXTR_SKIP);
+
+  // Start output buffering
+  ob_start();
+
+  // Include the template file
+  include \Drupal::root() . '/' . $template_file;
+
+  // End buffering and return its contents
+  return ob_get_clean();
+}

core/themes/engines/phptemplate/phptemplate.info.yml

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+type: theme_engine
+name: PHPTemplate
+core: 8.x
+version: VERSION
+package: Core