diff --git a/core/includes/batch.inc.php b/core/includes/batch.inc.php
index ef960e7..7d905cd 100644
--- a/core/includes/batch.inc.php
+++ b/core/includes/batch.inc.php
@@ -231,8 +231,8 @@ function _batch_process() {
     // If this is the first time we iterate this batch set in the current
     // request, we check if it requires an additional file for functions
     // definitions.
-    if ($set_changed && isset($current_set['file']) && is_file($current_set['file'] . '.php')) {
-      include_once \Drupal::root() . '/' . $current_set['file'] . '.php';
+    if ($set_changed && isset($current_set['file']) && is_file($current_set['file'])) {
+      include_once \Drupal::root() . '/' . $current_set['file'];
     }
 
     $task_message = $label = '';
@@ -404,8 +404,8 @@ function _batch_finished() {
   foreach ($batch['sets'] as $batch_set) {
     if (isset($batch_set['finished'])) {
       // Check if the set requires an additional file for function definitions.
-      if (isset($batch_set['file']) && is_file($batch_set['file'] . '.php')) {
-        include_once \Drupal::root() . '/' . $batch_set['file'] . '.php';
+      if (isset($batch_set['file']) && is_file($batch_set['file'])) {
+        include_once \Drupal::root() . '/' . $batch_set['file'];
       }
       if (is_callable($batch_set['finished'])) {
         $queue = _batch_queue($batch_set);
diff --git a/core/includes/install.core.inc.php b/core/includes/install.core.inc.php
index 25d81ea..5bed64b 100644
--- a/core/includes/install.core.inc.php
+++ b/core/includes/install.core.inc.php
@@ -1714,7 +1714,7 @@ function install_import_translations(&$install_state) {
       'progress_message' => '',
       'error_message' => t('Error importing translation files'),
       'finished' => 'locale_translation_batch_fetch_finished',
-      'file' => drupal_get_path('module', 'locale') . '/locale.batch.inc',
+      'file' => drupal_get_path('module', 'locale') . '/locale.batch.inc.php',
     );
     return $batch;
   }
diff --git a/core/includes/theme.inc.php b/core/includes/theme.inc.php
index b99fc6c..732f356 100644
--- a/core/includes/theme.inc.php
+++ b/core/includes/theme.inc.php
@@ -1851,13 +1851,13 @@ function drupal_common_theme() {
       'variables' => array('message' => NULL, 'success' => TRUE),
       'function' => 'theme_authorize_message',
       'path' => 'core/includes',
-      'file' => 'theme.maintenance.inc',
+      'file' => 'theme.maintenance.inc.php',
     ),
     'authorize_report' => array(
       'variables' => array('messages' => array()),
       'function' => 'theme_authorize_report',
       'path' => 'core/includes',
-      'file' => 'theme.maintenance.inc',
+      'file' => 'theme.maintenance.inc.php',
     ),
     // From pager.inc.
     'pager' => array(
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 5ba2972..805c9a9 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -431,7 +431,7 @@ protected function processExtension(&$cache, $name, $type, $theme, $path) {
         if (isset($info['file'])) {
           $include_file = isset($info['path']) ? $info['path'] : $path;
           $include_file .= '/' . $info['file'];
-          include_once $this->root . '/' . $include_file . '.php';
+          include_once $this->root . '/' . $include_file;
           $result[$hook]['includes'][] = $include_file;
         }
 
diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index ef1af0e..a100374 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -261,7 +261,7 @@ protected function theme($hook, $variables = array()) {
     // elsewhere.
     if (!empty($info['includes'])) {
       foreach ($info['includes'] as $include_file) {
-        include_once $this->root . '/' . $include_file . '.php';
+        include_once $this->root . '/' . $include_file;
       }
     }
 
@@ -273,7 +273,7 @@ protected function theme($hook, $variables = array()) {
       // preprocessors might reside there.
       if (!empty($base_hook_info['includes'])) {
         foreach ($base_hook_info['includes'] as $include_file) {
-          include_once $this->root . '/' . $include_file . '.php';
+          include_once $this->root . '/' . $include_file;
         }
       }
       // Replace the preprocess functions with those from the base hook.
diff --git a/core/modules/aggregator/aggregator.module.php b/core/modules/aggregator/aggregator.module.php
index 42002b1..b0e4cd4 100644
--- a/core/modules/aggregator/aggregator.module.php
+++ b/core/modules/aggregator/aggregator.module.php
@@ -64,15 +64,15 @@ function aggregator_theme() {
   return array(
     'aggregator_feed' => array(
       'render element' => 'elements',
-      'file' => 'aggregator.theme.inc',
+      'file' => 'aggregator.theme.inc.php',
     ),
     'aggregator_block_item' => array(
       'variables' => array('item' => NULL, 'feed' => 0),
-      'file' => 'aggregator.theme.inc',
+      'file' => 'aggregator.theme.inc.php',
     ),
     'aggregator_item' => array(
       'render element' => 'elements',
-      'file' => 'aggregator.theme.inc',
+      'file' => 'aggregator.theme.inc.php',
     ),
   );
 }
diff --git a/core/modules/block_content/block_content.module.php b/core/modules/block_content/block_content.module.php
index 7480c31..50cd1dd 100644
--- a/core/modules/block_content/block_content.module.php
+++ b/core/modules/block_content/block_content.module.php
@@ -46,7 +46,7 @@ function block_content_theme($existing, $type, $theme, $path) {
   return array(
     'block_content_add_list' => array(
       'variables' => array('content' => NULL),
-      'file' => 'block_content.pages.inc',
+      'file' => 'block_content.pages.inc.php',
     ),
   );
 }
diff --git a/core/modules/ckeditor/ckeditor.module.php b/core/modules/ckeditor/ckeditor.module.php
index 613ab1b..b0c319c 100644
--- a/core/modules/ckeditor/ckeditor.module.php
+++ b/core/modules/ckeditor/ckeditor.module.php
@@ -38,7 +38,7 @@ function ckeditor_help($route_name, RouteMatchInterface $route_match) {
 function ckeditor_theme() {
   return array(
     'ckeditor_settings_toolbar' => array(
-      'file' => 'ckeditor.admin.inc',
+      'file' => 'ckeditor.admin.inc.php',
       'variables' => array('editor' => NULL, 'plugins' => NULL),
     ),
   );
diff --git a/core/modules/config/src/Form/ConfigSync.php b/core/modules/config/src/Form/ConfigSync.php
index 2742e93..9c9923b 100644
--- a/core/modules/config/src/Form/ConfigSync.php
+++ b/core/modules/config/src/Form/ConfigSync.php
@@ -329,7 +329,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
           'init_message' => t('Starting configuration synchronization.'),
           'progress_message' => t('Completed @current step of @total.'),
           'error_message' => t('Configuration synchronization has encountered an error.'),
-          'file' => drupal_get_path('module', 'config') . '/config.admin.inc',
+          'file' => drupal_get_path('module', 'config') . '/config.admin.inc.php',
         );
         foreach ($sync_steps as $sync_step) {
           $batch['operations'][] = array(array(get_class($this), 'processBatch'), array($config_importer, $sync_step));
diff --git a/core/modules/file/file.module.php b/core/modules/file/file.module.php
index 85a9cd2..18e277f 100644
--- a/core/modules/file/file.module.php
+++ b/core/modules/file/file.module.php
@@ -558,15 +558,15 @@ function file_theme() {
     // From file.field.inc.
     'file_widget' => array(
       'render element' => 'element',
-      'file' => 'file.field.inc',
+      'file' => 'file.field.inc.php',
     ),
     'file_widget_multiple' => array(
       'render element' => 'element',
-      'file' => 'file.field.inc',
+      'file' => 'file.field.inc.php',
     ),
     'file_upload_help' => array(
       'variables' => array('description' => NULL, 'upload_validators' => NULL, 'cardinality' => NULL),
-      'file' => 'file.field.inc',
+      'file' => 'file.field.inc.php',
     ),
   );
 }
diff --git a/core/modules/image/image.module.php b/core/modules/image/image.module.php
index 1433eb1..4e2b1d9 100644
--- a/core/modules/image/image.module.php
+++ b/core/modules/image/image.module.php
@@ -112,14 +112,14 @@ function image_theme() {
       ),
     ),
 
-    // Theme functions in image.admin.inc.
+    // Theme functions in image.admin.inc.php.
     'image_style_preview' => array(
       'variables' => array('style' => NULL),
-      'file' => 'image.admin.inc',
+      'file' => 'image.admin.inc.php',
     ),
     'image_anchor' => array(
       'render element' => 'element',
-      'file' => 'image.admin.inc',
+      'file' => 'image.admin.inc.php',
     ),
     'image_resize_summary' => array(
       'variables' => array('data' => NULL, 'effect' => array()),
@@ -134,14 +134,14 @@ function image_theme() {
       'variables' => array('data' => NULL, 'effect' => array()),
     ),
 
-    // Theme functions in image.field.inc.
+    // Theme functions in image.field.inc.php.
     'image_widget' => array(
       'render element' => 'element',
-      'file' => 'image.field.inc',
+      'file' => 'image.field.inc.php',
     ),
     'image_formatter' => array(
       'variables' => array('item' => NULL, 'item_attributes' => NULL, 'url' => NULL, 'image_style' => NULL),
-      'file' => 'image.field.inc',
+      'file' => 'image.field.inc.php',
     ),
   );
 }
diff --git a/core/modules/language/language.module.php b/core/modules/language/language.module.php
index 5b2099e..905b663 100644
--- a/core/modules/language/language.module.php
+++ b/core/modules/language/language.module.php
@@ -102,16 +102,16 @@ function language_theme() {
   return array(
     'language_negotiation_configure_form' => array(
       'render element' => 'form',
-      'file' => 'language.admin.inc',
+      'file' => 'language.admin.inc.php',
     ),
     'language_negotiation_configure_browser_form_table' => array(
       'render element' => 'form',
-      'file' => 'language.admin.inc',
+      'file' => 'language.admin.inc.php',
       'function' => 'theme_language_negotiation_configure_browser_form_table',
     ),
     'language_content_settings_table' => array(
       'render element' => 'element',
-      'file' => 'language.admin.inc',
+      'file' => 'language.admin.inc.php',
       'function' => 'theme_language_content_settings_table',
     ),
   );
diff --git a/core/modules/locale/locale.bulk.inc.php b/core/modules/locale/locale.bulk.inc.php
index 357aaf8..afac73e 100644
--- a/core/modules/locale/locale.bulk.inc.php
+++ b/core/modules/locale/locale.bulk.inc.php
@@ -154,7 +154,7 @@ function locale_translate_batch_build(array $files, array $options) {
       'title'         => t('Importing interface translations'),
       'progress_message' => '',
       'error_message' => t('Error importing interface translations'),
-      'file'          => drupal_get_path('module', 'locale') . '/locale.bulk.inc',
+      'file'          => drupal_get_path('module', 'locale') . '/locale.bulk.inc.php',
     );
     if ($options['finish_feedback']) {
       $batch['finished'] = 'locale_translate_batch_finished';
@@ -580,7 +580,7 @@ function locale_config_batch_build(array $names, array $langcodes, array $option
     'title'         => t('Updating configuration translations'),
     'init_message'  => t('Starting configuration update'),
     'error_message' => t('Error updating configuration translations'),
-    'file'          => drupal_get_path('module', 'locale') . '/locale.bulk.inc',
+    'file'          => drupal_get_path('module', 'locale') . '/locale.bulk.inc.php',
   );
   if (!empty($options['finish_feedback'])) {
     $batch['completed'] = 'locale_config_batch_finished';
diff --git a/core/modules/locale/locale.compare.inc.php b/core/modules/locale/locale.compare.inc.php
index 471ed34..48eda61 100644
--- a/core/modules/locale/locale.compare.inc.php
+++ b/core/modules/locale/locale.compare.inc.php
@@ -266,7 +266,7 @@ function locale_translation_batch_status_build($projects = array(), $langcodes =
     'progress_message' => '',
     'finished' => 'locale_translation_batch_status_finished',
     'error_message' => t('Error checking translation updates.'),
-    'file' => drupal_get_path('module', 'locale') . '/locale.batch.inc',
+    'file' => drupal_get_path('module', 'locale') . '/locale.batch.inc.php',
   );
   return $batch;
 }
diff --git a/core/modules/locale/locale.fetch.inc.php b/core/modules/locale/locale.fetch.inc.php
index 99e9e68..bf73ebc 100644
--- a/core/modules/locale/locale.fetch.inc.php
+++ b/core/modules/locale/locale.fetch.inc.php
@@ -44,7 +44,7 @@ function locale_translation_batch_update_build($projects = array(), $langcodes =
     'progress_message' => '',
     'error_message' => t('Error importing translation files'),
     'finished' => 'locale_translation_batch_fetch_finished',
-    'file' => drupal_get_path('module', 'locale') . '/locale.batch.inc',
+    'file' => drupal_get_path('module', 'locale') . '/locale.batch.inc.php',
   );
   return $batch;
 }
@@ -73,7 +73,7 @@ function locale_translation_batch_fetch_build($projects = array(), $langcodes =
     'progress_message' => '',
     'error_message' => t('Error importing translation files'),
     'finished' => 'locale_translation_batch_fetch_finished',
-    'file' => drupal_get_path('module', 'locale') . '/locale.batch.inc',
+    'file' => drupal_get_path('module', 'locale') . '/locale.batch.inc.php',
   );
   return $batch;
 }
diff --git a/core/modules/locale/locale.module.php b/core/modules/locale/locale.module.php
index 0b07f54..c9cd41e 100644
--- a/core/modules/locale/locale.module.php
+++ b/core/modules/locale/locale.module.php
@@ -183,11 +183,11 @@ function locale_theme() {
   return array(
     'locale_translation_last_check' => array(
       'variables' => array('last' => NULL),
-      'file' => 'locale.pages.inc',
+      'file' => 'locale.pages.inc.php',
     ),
     'locale_translation_update_info' => array(
       'variables' => array('updates' => array(), 'not_found' => array()),
-      'file' => 'locale.pages.inc',
+      'file' => 'locale.pages.inc.php',
     ),
   );
 }
diff --git a/core/modules/node/node.admin.inc.php b/core/modules/node/node.admin.inc.php
index 053f88f..5da3ce7 100644
--- a/core/modules/node/node.admin.inc.php
+++ b/core/modules/node/node.admin.inc.php
@@ -47,7 +47,7 @@ function node_mass_update(array $nodes, array $updates, $langcode = NULL, $load
       'error_message' => t('The update has encountered an error.'),
       // The operations do not live in the .module file, so we need to
       // tell the batch engine which file to load before calling them.
-      'file' => drupal_get_path('module', 'node') . '/node.admin.inc',
+      'file' => drupal_get_path('module', 'node') . '/node.admin.inc.php',
     );
     batch_set($batch);
   }
diff --git a/core/modules/node/node.module.php b/core/modules/node/node.module.php
index d5dfba1..dc9bdcf 100644
--- a/core/modules/node/node.module.php
+++ b/core/modules/node/node.module.php
@@ -137,7 +137,7 @@ function node_theme() {
     ),
     'node_add_list' => array(
       'variables' => array('content' => NULL),
-      'file' => 'node.pages.inc',
+      'file' => 'node.pages.inc.php',
     ),
     'node_edit_form' => array(
       'render element' => 'form',
diff --git a/core/modules/search/search.module.php b/core/modules/search/search.module.php
index e05576d..7538308 100644
--- a/core/modules/search/search.module.php
+++ b/core/modules/search/search.module.php
@@ -103,7 +103,7 @@ function search_theme() {
   return array(
     'search_result' => array(
       'variables' => array('result' => NULL, 'plugin_id' => NULL),
-      'file' => 'search.pages.inc',
+      'file' => 'search.pages.inc.php',
     ),
   );
 }
diff --git a/core/modules/system/src/Tests/FileTransfer/FileTransferTest.php b/core/modules/system/src/Tests/FileTransfer/FileTransferTest.php
index 07ce6ad..4f44cdc 100644
--- a/core/modules/system/src/Tests/FileTransfer/FileTransferTest.php
+++ b/core/modules/system/src/Tests/FileTransfer/FileTransferTest.php
@@ -30,13 +30,13 @@ protected function setUp() {
 
   function _getFakeModuleFiles() {
     $files = array(
-      'fake.module',
+      'fake.module.php',
       'fake.info.yml',
       'theme' => array(
         'fake.html.twig'
       ),
       'inc' => array(
-        'fake.inc'
+        'fake.inc.php'
       )
     );
     return $files;
diff --git a/core/modules/system/system.module.php b/core/modules/system/system.module.php
index 8a55125..32150b9 100644
--- a/core/modules/system/system.module.php
+++ b/core/modules/system/system.module.php
@@ -171,7 +171,7 @@ function system_theme() {
         'theme_groups' => array(),
         'theme_group_titles' => array(),
       ),
-      'file' => 'system.admin.inc',
+      'file' => 'system.admin.inc.php',
     ),
     'system_config_form' => array(
       'render element' => 'form',
@@ -181,33 +181,33 @@ function system_theme() {
     ),
     'system_modules_details' => array(
       'render element' => 'form',
-      'file' => 'system.admin.inc',
+      'file' => 'system.admin.inc.php',
       'function' => 'theme_system_modules_details',
     ),
     'system_modules_uninstall' => array(
       'render element' => 'form',
-      'file' => 'system.admin.inc',
+      'file' => 'system.admin.inc.php',
       'function' => 'theme_system_modules_uninstall',
     ),
     'status_report' => array(
       'variables' => array('requirements' => NULL),
-      'file' => 'system.admin.inc',
+      'file' => 'system.admin.inc.php',
     ),
     'admin_page' => array(
       'variables' => array('blocks' => NULL),
-      'file' => 'system.admin.inc',
+      'file' => 'system.admin.inc.php',
     ),
     'admin_block' => array(
       'variables' => array('block' => NULL),
-      'file' => 'system.admin.inc',
+      'file' => 'system.admin.inc.php',
     ),
     'admin_block_content' => array(
       'variables' => array('content' => NULL),
-      'file' => 'system.admin.inc',
+      'file' => 'system.admin.inc.php',
     ),
     'system_admin_index' => array(
       'variables' => array('menu_items' => NULL),
-      'file' => 'system.admin.inc',
+      'file' => 'system.admin.inc.php',
     ),
   ));
 }
@@ -373,7 +373,7 @@ function _system_themes_access($theme) {
  * Example (update_manager_install_form_submit()):
  * @code
  *  system_authorized_init('update_authorize_run_install',
- *    drupal_get_path('module', 'update') . '/update.authorize.inc',
+ *    drupal_get_path('module', 'update') . '/update.authorize.inc.php',
  *    $arguments, t('Update manager'));
  *  $form_state->setRedirectUrl(system_authorized_get_url());
  * @endcode
diff --git a/core/modules/system/tests/modules/batch_test/batch_test.module.php b/core/modules/system/tests/modules/batch_test/batch_test.module.php
index d4bbcc7..3f874b1 100644
--- a/core/modules/system/tests/modules/batch_test/batch_test.module.php
+++ b/core/modules/system/tests/modules/batch_test/batch_test.module.php
@@ -23,7 +23,7 @@ function _batch_test_batch_0() {
   $batch = array(
     'operations' => array(),
     'finished' => '_batch_test_finished_0',
-    'file' => drupal_get_path('module', 'batch_test'). '/batch_test.callbacks.inc',
+    'file' => drupal_get_path('module', 'batch_test'). '/batch_test.callbacks.inc.php',
   );
   return $batch;
 }
@@ -45,7 +45,7 @@ function _batch_test_batch_1() {
   $batch = array(
     'operations' => $operations,
     'finished' => '_batch_test_finished_1',
-    'file' => drupal_get_path('module', 'batch_test'). '/batch_test.callbacks.inc',
+    'file' => drupal_get_path('module', 'batch_test'). '/batch_test.callbacks.inc.php',
   );
   return $batch;
 }
@@ -66,7 +66,7 @@ function _batch_test_batch_2() {
   $batch = array(
     'operations' => $operations,
     'finished' => '_batch_test_finished_2',
-    'file' => drupal_get_path('module', 'batch_test') . '/batch_test.callbacks.inc',
+    'file' => drupal_get_path('module', 'batch_test') . '/batch_test.callbacks.inc.php',
   );
   return $batch;
 }
@@ -97,7 +97,7 @@ function _batch_test_batch_3() {
   $batch = array(
     'operations' => $operations,
     'finished' => '_batch_test_finished_3',
-    'file' => drupal_get_path('module', 'batch_test') . '/batch_test.callbacks.inc',
+    'file' => drupal_get_path('module', 'batch_test') . '/batch_test.callbacks.inc.php',
   );
   return $batch;
 }
@@ -126,7 +126,7 @@ function _batch_test_batch_4() {
   $batch = array(
     'operations' => $operations,
     'finished' => '_batch_test_finished_4',
-    'file' => drupal_get_path('module', 'batch_test') . '/batch_test.callbacks.inc',
+    'file' => drupal_get_path('module', 'batch_test') . '/batch_test.callbacks.inc.php',
   );
   return $batch;
 }
@@ -148,7 +148,7 @@ function _batch_test_batch_5() {
   $batch = array(
     'operations' => $operations,
     'finished' => '_batch_test_finished_5',
-    'file' => drupal_get_path('module', 'batch_test'). '/batch_test.callbacks.inc',
+    'file' => drupal_get_path('module', 'batch_test'). '/batch_test.callbacks.inc.php',
   );
   return $batch;
 }
diff --git a/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module.php b/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module.php
index 974856a..a7f25be 100644
--- a/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module.php
+++ b/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module.php
@@ -10,7 +10,7 @@
  */
 function theme_suggestions_test_theme() {
   $items['theme_suggestions_test_include'] = array(
-    'file' => 'theme_suggestions_test.inc',
+    'file' => 'theme_suggestions_test.inc.php',
     'variables' => array(),
     'function' => 'theme_theme_suggestions_test_include',
   );
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module.php b/core/modules/system/tests/modules/theme_test/theme_test.module.php
index 7ba5c2d..f54f906 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module.php
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module.php
@@ -7,7 +7,7 @@
  */
 function theme_test_theme($existing, $type, $theme, $path) {
   $items['theme_test'] = array(
-    'file' => 'theme_test.inc',
+    'file' => 'theme_test.inc.php',
     'variables' => array('foo' => ''),
     'function' => 'theme_theme_test',
   );
diff --git a/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
index b88f740..3e5b17d 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -48,7 +48,7 @@
  *       'result' => NULL,
  *       'plugin_id' => NULL,
  *     ),
- *    'file' => 'search.pages.inc',
+ *    'file' => 'search.pages.inc.php',
  *   ),
  * );
  * @endcode
@@ -1072,7 +1072,7 @@ function hook_theme($existing, $type, $theme, $path) {
     ),
     'status_report' => array(
       'render element' => 'requirements',
-      'file' => 'system.admin.inc',
+      'file' => 'system.admin.inc.php',
     ),
   );
 }
@@ -1098,7 +1098,7 @@ function hook_theme($existing, $type, $theme, $path) {
  *     'account' => NULL,
  *   ),
  *   'template' => 'core/modules/user/user',
- *   'file' => 'core/modules/user/user.pages.inc',
+ *   'file' => 'core/modules/user/user.pages.inc.php',
  *   'type' => 'module',
  *   'theme path' => 'core/modules/user',
  *   'preprocess functions' => array(
diff --git a/core/modules/update/src/Form/UpdateManagerInstall.php b/core/modules/update/src/Form/UpdateManagerInstall.php
index b3e2deb..9d8495c 100644
--- a/core/modules/update/src/Form/UpdateManagerInstall.php
+++ b/core/modules/update/src/Form/UpdateManagerInstall.php
@@ -226,7 +226,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
     // credentials and invoke update_authorize_run_install() indirectly with
     // whatever FileTransfer object authorize.php creates for us.
     else {
-      system_authorized_init('update_authorize_run_install', drupal_get_path('module', 'update') . '/update.authorize.inc', $arguments, $this->t('Update manager'));
+      system_authorized_init('update_authorize_run_install', drupal_get_path('module', 'update') . '/update.authorize.inc.php', $arguments, $this->t('Update manager'));
       $form_state->setRedirectUrl(system_authorized_get_url());
     }
   }
diff --git a/core/modules/update/src/Form/UpdateManagerUpdate.php b/core/modules/update/src/Form/UpdateManagerUpdate.php
index ce342e1..f2617b2 100644
--- a/core/modules/update/src/Form/UpdateManagerUpdate.php
+++ b/core/modules/update/src/Form/UpdateManagerUpdate.php
@@ -338,7 +338,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       'init_message' => $this->t('Preparing to download selected updates'),
       'operations' => $operations,
       'finished' => 'update_manager_download_batch_finished',
-      'file' => drupal_get_path('module', 'update') . '/update.manager.inc',
+      'file' => drupal_get_path('module', 'update') . '/update.manager.inc.php',
     );
     batch_set($batch);
   }
diff --git a/core/modules/update/src/Form/UpdateReady.php b/core/modules/update/src/Form/UpdateReady.php
index 8e106fe..44cca18 100644
--- a/core/modules/update/src/Form/UpdateReady.php
+++ b/core/modules/update/src/Form/UpdateReady.php
@@ -151,7 +151,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       // credentials and invoke update_authorize_run_update() indirectly with
       // whatever FileTransfer object authorize.php creates for us.
       else {
-        system_authorized_init('update_authorize_run_update', drupal_get_path('module', 'update') . '/update.authorize.inc', array($updates), $this->t('Update manager'));
+        system_authorized_init('update_authorize_run_update', drupal_get_path('module', 'update') . '/update.authorize.inc.php', array($updates), $this->t('Update manager'));
         $form_state->setRedirectUrl(system_authorized_get_url());
       }
     }
diff --git a/core/modules/update/update.authorize.inc.php b/core/modules/update/update.authorize.inc.php
index d6a7066..a7e2d70 100644
--- a/core/modules/update/update.authorize.inc.php
+++ b/core/modules/update/update.authorize.inc.php
@@ -48,7 +48,7 @@ function update_authorize_run_update($filetransfer, $projects) {
     'init_message' => t('Preparing to update your site'),
     'operations' => $operations,
     'finished' => 'update_authorize_update_batch_finished',
-    'file' => drupal_get_path('module', 'update') . '/update.authorize.inc',
+    'file' => drupal_get_path('module', 'update') . '/update.authorize.inc.php',
   );
 
   batch_set($batch);
@@ -93,7 +93,7 @@ function update_authorize_run_install($filetransfer, $project, $updater_name, $l
     'operations' => $operations,
     // @todo Use a different finished callback for different messages?
     'finished' => 'update_authorize_install_batch_finished',
-    'file' => drupal_get_path('module', 'update') . '/update.authorize.inc',
+    'file' => drupal_get_path('module', 'update') . '/update.authorize.inc.php',
   );
   batch_set($batch);
 
diff --git a/core/modules/update/update.module.php b/core/modules/update/update.module.php
index 99dad70..7385e24 100644
--- a/core/modules/update/update.module.php
+++ b/core/modules/update/update.module.php
@@ -197,17 +197,17 @@ function update_theme() {
     ),
     'update_report' => array(
       'variables' => array('data' => NULL),
-      'file' => 'update.report.inc',
+      'file' => 'update.report.inc.php',
     ),
     'update_project_status' => array(
       'variables' => array('project' => array(), 'includes_status' => array()),
-      'file' => 'update.report.inc',
+      'file' => 'update.report.inc.php',
     ),
     // We are using template instead of '#type' => 'table' here to keep markup
     // out of preprocess and allow for easier changes to markup.
     'update_version' => array(
       'variables' => array('version' => NULL, 'title' => NULL, 'attributes' => array()),
-      'file' => 'update.report.inc',
+      'file' => 'update.report.inc.php',
     ),
   );
 }
diff --git a/core/modules/views/views.module.php b/core/modules/views/views.module.php
index 1b66056..ea5d8b2 100644
--- a/core/modules/views/views.module.php
+++ b/core/modules/views/views.module.php
@@ -88,7 +88,7 @@ function views_theme($existing, $type, $theme, $path) {
 
   // Some quasi clever array merging here.
   $base = array(
-    'file' => 'views.theme.inc',
+    'file' => 'views.theme.inc.php',
   );
 
   // Our extra version of pager from pager.inc
@@ -168,8 +168,8 @@ function views_theme($existing, $type, $theme, $path) {
         if (!isset($hooks[$def['theme']]['includes'])) {
           $hooks[$def['theme']]['includes'] = array();
         }
-        if (!in_array('views.theme.inc', $hooks[$def['theme']]['includes'])) {
-          $hooks[$def['theme']]['includes'][] = $module_dir . '/views.theme.inc';
+        if (!in_array('views.theme.inc.php', $hooks[$def['theme']]['includes'])) {
+          $hooks[$def['theme']]['includes'][] = $module_dir . '/views.theme.inc.php';
         }
       }
       // The theme_file definition is always relative to the modules directory.
@@ -180,7 +180,7 @@ function views_theme($existing, $type, $theme, $path) {
       // Whenever we have a theme file, we include it directly so we can
       // auto-detect the theme function.
       if (isset($def['theme_file'])) {
-        $include = \Drupal::root() . '/' . $module_dir. '/' . $def['theme_file'] . '.php';
+        $include = \Drupal::root() . '/' . $module_dir. '/' . $def['theme_file'];
         if (is_file($include)) {
           require_once $include;
         }
@@ -220,7 +220,7 @@ function views_theme($existing, $type, $theme, $path) {
  * A theme preprocess function to automatically allow view-based node
  * templates if called from a view.
  *
- * The 'modules/node.views.inc' file is a better place for this, but
+ * The 'modules/node.views.inc.php' file is a better place for this, but
  * we haven't got a chance to load that file before Drupal builds the
  * node portion of the theme registry.
  */
diff --git a/core/modules/views_ui/views_ui.module.php b/core/modules/views_ui/views_ui.module.php
index f9f8996..3ee1cbb 100644
--- a/core/modules/views_ui/views_ui.module.php
+++ b/core/modules/views_ui/views_ui.module.php
@@ -72,51 +72,51 @@ function views_ui_theme() {
     // edit a view
     'views_ui_display_tab_setting' => array(
       'variables' => array('description' => '', 'link' => '', 'settings_links' => array(), 'overridden' => FALSE, 'defaulted' => FALSE, 'description_separator' => TRUE, 'class' => array()),
-      'file' => 'views_ui.theme.inc',
+      'file' => 'views_ui.theme.inc.php',
     ),
     'views_ui_display_tab_bucket' => array(
       'render element' => 'element',
-      'file' => 'views_ui.theme.inc',
+      'file' => 'views_ui.theme.inc.php',
     ),
     'views_ui_rearrange_filter_form' => array(
       'render element' => 'form',
-      'file' => 'views_ui.theme.inc',
+      'file' => 'views_ui.theme.inc.php',
     ),
     'views_ui_expose_filter_form' => array(
       'render element' => 'form',
-      'file' => 'views_ui.theme.inc',
+      'file' => 'views_ui.theme.inc.php',
     ),
 
     // list views
     'views_ui_view_info' => array(
       'variables' => array('view' => NULL, 'displays' => NULL),
-      'file' => 'views_ui.theme.inc',
+      'file' => 'views_ui.theme.inc.php',
     ),
 
     // Group of filters.
     'views_ui_build_group_filter_form' => array(
       'render element' => 'form',
-      'file' => 'views_ui.theme.inc',
+      'file' => 'views_ui.theme.inc.php',
       'function' => 'theme_views_ui_build_group_filter_form',
     ),
 
     // On behalf of a plugin
     'views_ui_style_plugin_table' => array(
       'render element' => 'form',
-      'file' => 'views_ui.theme.inc',
+      'file' => 'views_ui.theme.inc.php',
     ),
 
     // When previewing a view.
     'views_ui_view_preview_section' => array(
       'variables' => array('view' => NULL, 'section' => NULL, 'content' => NULL, 'links' => ''),
-      'file' => 'views_ui.theme.inc',
+      'file' => 'views_ui.theme.inc.php',
     ),
 
     // Generic container wrapper, to use instead of theme_container when an id
     // is not desired.
     'views_ui_container' => array(
       'variables' => array('children' => NULL, 'attributes' => array()),
-      'file' => 'views_ui.theme.inc',
+      'file' => 'views_ui.theme.inc.php',
     ),
   );
 }