diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 6fc242b..e89cb8f 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -268,6 +268,10 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
     }
   }
 
+  // Always include Twig so that PHPTemplate themes do not have to convert all
+  // core and contrib templates in order to use the PHPTemplate engine.
+  include_once DRUPAL_ROOT . '/core/themes/engines/twig/twig.engine';
+
   if (isset($registry_callback)) {
     _theme_registry_callback($registry_callback, array($theme, $base_theme, $theme_engine));
   }
@@ -1086,10 +1090,8 @@ function theme($hook, $variables = array()) {
     }
   }
   else {
-    // Include Twig, the default theme engine.
-    include_once DRUPAL_ROOT . '/core/themes/engines/twig/twig.engine';
     $render_function = 'twig_render_template';
-    $extension = twig_extension();
+    $extension = '.html.twig';
 
     // The theme engine may use a different extension and a different renderer.
     global $theme_engine;
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
index 0a1ab10..135a435 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
@@ -31,7 +31,7 @@ public static function getInfo() {
 
   function setUp() {
     parent::setUp();
-    theme_enable(array('test_theme_twig'));
+    theme_enable(array('test_theme'));
   }
 
   /**
@@ -39,7 +39,7 @@ function setUp() {
    */
   function testTemplateOverride() {
     config('system.theme')
-      ->set('default', 'test_theme_twig')
+      ->set('default', 'test_theme')
       ->save();
     $this->drupalGet('theme-test/template-test');
     $this->assertText('Success: Template overridden.', t('Template overridden by defined \'template\' filename.'));
@@ -50,7 +50,7 @@ function testTemplateOverride() {
    */
   function testTwigVariableDataTypes() {
     config('system.theme')
-      ->set('default', 'test_theme_twig')
+      ->set('default', 'test_theme')
       ->save();
     $this->drupalGet('twig-theme-test/php-variables');
     foreach (_test_theme_twig_php_values() as $type => $value) {
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
index 0bcd1a6..38e183b 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
@@ -34,8 +34,8 @@ public static function getInfo() {
    */
   function testTwigDebugMarkup() {
     $extension = twig_extension();
-    theme_enable(array('test_theme_twig'));
-    config('system.theme')->set('default', 'test_theme_twig')->save();
+    theme_enable(array('test_theme'));
+    config('system.theme')->set('default', 'test_theme')->save();
     // Enable debug, rebuild the service container, and clear all caches.
     $this->settingsSet('twig_debug', TRUE);
     $this->rebuildContainer();
@@ -47,7 +47,7 @@ function testTwigDebugMarkup() {
       _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
     }
     // Create array of Twig templates.
-    $templates = drupal_find_theme_templates($cache, $extension, drupal_get_path('theme', 'test_theme_twig'));
+    $templates = drupal_find_theme_templates($cache, $extension, drupal_get_path('theme', 'test_theme'));
     $templates += drupal_find_theme_templates($cache, $extension, drupal_get_path('module', 'node'));
 
     // Create a node and test different features of the debug markup.
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
index 2e84924..f50f110 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
@@ -77,9 +77,9 @@ function testTwigDebugOverride() {
    */
   function testTwigCacheOverride() {
     $extension = twig_extension();
-    theme_enable(array('test_theme_twig'));
+    theme_enable(array('test_theme'));
     config('system.theme')
-      ->set('default', 'test_theme_twig')
+      ->set('default', 'test_theme')
       ->save();
 
     $cache = array();
@@ -89,7 +89,7 @@ function testTwigCacheOverride() {
     }
 
     // Load array of Twig templates.
-    $templates = drupal_find_theme_templates($cache, $extension, drupal_get_path('theme', 'test_theme_twig'));
+    $templates = drupal_find_theme_templates($cache, $extension, drupal_get_path('theme', 'test_theme'));
 
     // Get the template filename and the cache filename for
     // theme_test.template_test.html.twig.
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.html.twig
new file mode 100644
index 0000000..cde8faa
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.html.twig
@@ -0,0 +1,2 @@
+<!-- Output for Theme API test -->
+Fail: Template not overridden.
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.tpl.php b/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.tpl.php
deleted file mode 100644
index cde8faa..0000000
--- a/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.tpl.php
+++ /dev/null
@@ -1,2 +0,0 @@
-<!-- Output for Theme API test -->
-Fail: Template not overridden.
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 137fdfc..e72334c 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -27,7 +27,7 @@ function theme_test_system_theme_info() {
   $themes['test_theme'] = drupal_get_path('module', 'system') . '/tests/themes/test_theme/test_theme.info.yml';
   $themes['test_basetheme'] = drupal_get_path('module', 'system') . '/tests/themes/test_basetheme/test_basetheme.info.yml';
   $themes['test_subtheme'] = drupal_get_path('module', 'system') . '/tests/themes/test_subtheme/test_subtheme.info.yml';
-  $themes['test_theme_twig'] = drupal_get_path('module', 'system') . '/tests/themes/test_theme_twig/test_theme_twig.info.yml';
+  $themes['test_theme_phptemplate'] = drupal_get_path('module', 'system') . '/tests/themes/test_theme_phptemplate/test_theme_phptemplate.info.yml';
   return $themes;
 }
 
diff --git a/core/modules/system/tests/themes/test_theme/node--1.html.twig b/core/modules/system/tests/themes/test_theme/node--1.html.twig
new file mode 100644
index 0000000..046c02e
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/node--1.html.twig
@@ -0,0 +1,4 @@
+{#
+  // node--1.html.twig - Dummy file for finding the template
+#}
+Node Content Dummy
diff --git a/core/modules/system/tests/themes/test_theme/node--1.tpl.php b/core/modules/system/tests/themes/test_theme/node--1.tpl.php
deleted file mode 100644
index 81974ce..0000000
--- a/core/modules/system/tests/themes/test_theme/node--1.tpl.php
+++ /dev/null
@@ -1,4 +0,0 @@
-<?php
-  // node--1.tpl.php - Dummy file for finding the template
-?>
-Node Content Dummy
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index 8275818..62b5abf 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -1,6 +1,18 @@
 <?php
 
 /**
+ * @file
+ * Theme to help test the Twig engine.
+ */
+
+/**
+ * Implements THEME_preprocess_twig_theme_test_php_variables().
+ */
+function test_theme_preprocess_twig_theme_test_php_variables(&$variables) {
+  $variables['php_values'] = _test_theme_twig_php_values();
+}
+
+/**
  * Tests a theme overriding a suggestion of a base theme hook.
  */
 function test_theme_theme_test__suggestion($variables) {
diff --git a/core/modules/system/tests/themes/test_theme/theme_test.template_test.html.twig b/core/modules/system/tests/themes/test_theme/theme_test.template_test.html.twig
new file mode 100644
index 0000000..6cda319
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/theme_test.template_test.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Success: Template overridden.
diff --git a/core/modules/system/tests/themes/test_theme/theme_test.template_test.tpl.php b/core/modules/system/tests/themes/test_theme/theme_test.template_test.tpl.php
deleted file mode 100644
index d4409bf..0000000
--- a/core/modules/system/tests/themes/test_theme/theme_test.template_test.tpl.php
+++ /dev/null
@@ -1,2 +0,0 @@
-<!-- Output for Theme API test -->
-Success: Template overridden.
diff --git a/core/modules/system/tests/themes/test_theme_phptemplate/node--1.tpl.php b/core/modules/system/tests/themes/test_theme_phptemplate/node--1.tpl.php
new file mode 100644
index 0000000..81974ce
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_phptemplate/node--1.tpl.php
@@ -0,0 +1,4 @@
+<?php
+  // node--1.tpl.php - Dummy file for finding the template
+?>
+Node Content Dummy
diff --git a/core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.info.yml b/core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.info.yml
new file mode 100644
index 0000000..fa975b9
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.info.yml
@@ -0,0 +1,6 @@
+name: 'Test theme PHPTemplate'
+type: theme
+description: 'Theme for testing the theme system with the PHPTemplate engine'
+core: 8.x
+engine: phptemplate
+hidden: true
diff --git a/core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.theme b/core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.theme
new file mode 100644
index 0000000..b3d9bbc
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.theme
@@ -0,0 +1 @@
+<?php
diff --git a/core/modules/system/tests/themes/test_theme_phptemplate/theme_test.template_test.tpl.php b/core/modules/system/tests/themes/test_theme_phptemplate/theme_test.template_test.tpl.php
new file mode 100644
index 0000000..d4409bf
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_phptemplate/theme_test.template_test.tpl.php
@@ -0,0 +1,2 @@
+<!-- Output for Theme API test -->
+Success: Template overridden.
diff --git a/core/modules/system/tests/themes/test_theme_twig/node--1.html.twig b/core/modules/system/tests/themes/test_theme_twig/node--1.html.twig
deleted file mode 100644
index 046c02e..0000000
--- a/core/modules/system/tests/themes/test_theme_twig/node--1.html.twig
+++ /dev/null
@@ -1,4 +0,0 @@
-{#
-  // node--1.html.twig - Dummy file for finding the template
-#}
-Node Content Dummy
diff --git a/core/modules/system/tests/themes/test_theme_twig/template.php b/core/modules/system/tests/themes/test_theme_twig/template.php
deleted file mode 100644
index a4abe2d..0000000
--- a/core/modules/system/tests/themes/test_theme_twig/template.php
+++ /dev/null
@@ -1,2 +0,0 @@
-<?php
-
diff --git a/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.info.yml b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.info.yml
deleted file mode 100644
index d2dd526..0000000
--- a/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.info.yml
+++ /dev/null
@@ -1,5 +0,0 @@
-name: 'Test theme for Twig'
-type: theme
-description: 'Theme for testing the theme system with twig engine'
-core: 8.x
-hidden: true
diff --git a/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
deleted file mode 100644
index 43aac12..0000000
--- a/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
+++ /dev/null
@@ -1,13 +0,0 @@
-<?php
-
-/**
- * @file
- * Theme to help test the Twig engine.
- */
-
-/**
- * Implements THEME_preprocess_twig_theme_test_php_variables().
- */
-function test_theme_twig_preprocess_twig_theme_test_php_variables(&$variables) {
-  $variables['php_values'] = _test_theme_twig_php_values();
-}
diff --git a/core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.html.twig b/core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.html.twig
deleted file mode 100644
index 6cda319..0000000
--- a/core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.html.twig
+++ /dev/null
@@ -1,2 +0,0 @@
-{# Output for Theme API test #}
-Success: Template overridden.
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index 1ba60a5..b541cdc 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -10,7 +10,9 @@
  * Implements hook_theme().
  */
 function twig_theme($existing, $type, $theme, $path) {
-  return drupal_find_theme_templates($existing, '.html.twig', $path);
+  $templates = drupal_find_theme_functions($existing, array($theme));
+  $templates += drupal_find_theme_templates($existing, '.html.twig', $path);
+  return $templates;
 }
 
 /**