diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 9ede033..4788bfc 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -21,7 +21,7 @@
 /**
  * Minimum supported version of PHP.
  *
- * @deprecated in Drupal 8.5.0, will be removed before Drupal 9.0.0. Use
+ * @deprecated in Drupal 8.6.0, will be removed before Drupal 9.0.0. Use
  *   \Drupal\Core\Requirements::DRUPAL_MINIMUM_PHP instead.
  *
  * @see https://www.drupal.org/node/2909361
@@ -44,7 +44,7 @@
  * contributed modules to be installed prior to hitting the limit. However,
  * 40M is the target for the Standard installation profile.
  *
- * @deprecated in Drupal 8.5.0, will be removed before Drupal 9.0.0. Use
+ * @deprecated in Drupal 8.6.0, will be removed before Drupal 9.0.0. Use
  *   \Drupal\Core\Requirements::DRUPAL_MINIMUM_PHP_MEMORY_LIMIT instead.
  *
  * @see https://www.drupal.org/node/2909361
diff --git a/core/includes/install.inc b/core/includes/install.inc
index 3529d51..2ae164c 100644
--- a/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -11,6 +11,7 @@
 use Drupal\Component\Utility\OpCodeCache;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Requirements;
 use Drupal\Core\Site\Settings;
 
 /**
@@ -1080,7 +1081,7 @@ function install_profile_info($profile, $langcode = 'en') {
       'description' => '',
       'version' => NULL,
       'hidden' => FALSE,
-      'php' => DRUPAL_MINIMUM_PHP,
+      'php' => Requirements::MINIMUM_PHP,
     ];
     $profile_file = drupal_get_path('profile', $profile) . "/$profile.info.yml";
     $info = \Drupal::service('info_parser')->parse($profile_file);
diff --git a/core/install.php b/core/install.php
index c88ffee..dad65da 100644
--- a/core/install.php
+++ b/core/install.php
@@ -6,6 +6,7 @@
  */
 
 use Drupal\Component\Utility\OpCodeCache;
+use Drupal\Core\Requirements;
 
 // Change the directory to the Drupal root.
 chdir('..');
@@ -21,11 +22,10 @@
  */
 define('MAINTENANCE_MODE', 'install');
 
-// Exit early if running an incompatible PHP version to avoid fatal errors.
-// The minimum version is specified explicitly, as DRUPAL_MINIMUM_PHP is not
-// yet available. It is defined in bootstrap.inc, but it is not possible to
-// load that file yet as it would cause a fatal error on older versions of PHP.
-if (version_compare(PHP_VERSION, '5.5.9') < 0) {
+// Checking minimum PHP version to avoid fatal errors.
+// The minimum version is specified in the Drupal\Core\Requirements interface
+require_once $root_path . '/core/lib/Drupal/Core/Requirements.php';
+if (version_compare(PHP_VERSION, Requirements::MINIMUM_PHP) < 0) {
   print 'Your PHP installation is too old. Drupal requires at least PHP 5.5.9. See the <a href="https://www.drupal.org/requirements">system requirements</a> page for more information.';
   exit;
 }
diff --git a/core/lib/Drupal/Core/Extension/module.api.php b/core/lib/Drupal/Core/Extension/module.api.php
index 45355cc..77b9eb2 100644
--- a/core/lib/Drupal/Core/Extension/module.api.php
+++ b/core/lib/Drupal/Core/Extension/module.api.php
@@ -6,6 +6,7 @@
  */
 
 use Drupal\Core\Database\Database;
+use Drupal\Core\Requirements;
 use Drupal\Core\Url;
 use Drupal\Core\Utility\UpdateException;
 
@@ -942,8 +943,8 @@ function hook_requirements($phase) {
     'title' => t('PHP'),
     'value' => ($phase == 'runtime') ? \Drupal::l(phpversion(), new Url('system.php')) : phpversion(),
   ];
-  if (version_compare(phpversion(), DRUPAL_MINIMUM_PHP) < 0) {
-    $requirements['php']['description'] = t('Your PHP installation is too old. Drupal requires at least PHP %version.', ['%version' => DRUPAL_MINIMUM_PHP]);
+  if (version_compare(phpversion(), Requirements::MINIMUM_PHP) < 0) {
+    $requirements['php']['description'] = t('Your PHP installation is too old. Drupal requires at least PHP %version.', ['%version' => Requirements::MINIMUM_PHP]);
     $requirements['php']['severity'] = REQUIREMENT_ERROR;
   }
 
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index 0fd4b26..9b04fe0 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -19,6 +19,7 @@
 use Drupal\Core\Entity\FieldableEntityInterface;
 use Drupal\Core\DrupalKernel;
 use Drupal\Core\Field\BaseFieldDefinition;
+use Drupal\Core\Requirements;
 use Drupal\Core\Site\Settings;
 use Drupal\Core\StreamWrapper\PrivateStream;
 use Drupal\Core\StreamWrapper\PublicStream;
@@ -183,8 +184,8 @@ function system_requirements($phase) {
     ];
   }
 
-  if (version_compare($phpversion, DRUPAL_MINIMUM_PHP) < 0) {
-    $requirements['php']['description'] = t('Your PHP installation is too old. Drupal requires at least PHP %version.', ['%version' => DRUPAL_MINIMUM_PHP]);
+  if (version_compare($phpversion, Requirements::MINIMUM_PHP) < 0) {
+    $requirements['php']['description'] = t('Your PHP installation is too old. Drupal requires at least PHP %version.', ['%version' => Requirements::MINIMUM_PHP]);
     $requirements['php']['severity'] = REQUIREMENT_ERROR;
     // If PHP is old, it's not safe to continue with the requirements check.
     return $requirements;
@@ -376,16 +377,16 @@ function system_requirements($phase) {
     'value' => $memory_limit == -1 ? t('-1 (Unlimited)') : $memory_limit,
   ];
 
-  if (!Environment::checkMemoryLimit(DRUPAL_MINIMUM_PHP_MEMORY_LIMIT, $memory_limit)) {
+  if (!Environment::checkMemoryLimit(Requirements::MINIMUM_PHP_MEMORY_LIMIT, $memory_limit)) {
     $description = [];
     if ($phase == 'install') {
-      $description['phase'] = t('Consider increasing your PHP memory limit to %memory_minimum_limit to help prevent errors in the installation process.', ['%memory_minimum_limit' => DRUPAL_MINIMUM_PHP_MEMORY_LIMIT]);
+      $description['phase'] = t('Consider increasing your PHP memory limit to %memory_minimum_limit to help prevent errors in the installation process.', ['%memory_minimum_limit' => Requirements::MINIMUM_PHP_MEMORY_LIMIT]);
     }
     elseif ($phase == 'update') {
-      $description['phase'] = t('Consider increasing your PHP memory limit to %memory_minimum_limit to help prevent errors in the update process.', ['%memory_minimum_limit' => DRUPAL_MINIMUM_PHP_MEMORY_LIMIT]);
+      $description['phase'] = t('Consider increasing your PHP memory limit to %memory_minimum_limit to help prevent errors in the update process.', ['%memory_minimum_limit' => Requirements::MINIMUM_PHP_MEMORY_LIMIT]);
     }
     elseif ($phase == 'runtime') {
-      $description['phase'] = t('Depending on your configuration, Drupal can run with a %memory_limit PHP memory limit. However, a %memory_minimum_limit PHP memory limit or above is recommended, especially if your site uses additional custom or contributed modules.', ['%memory_limit' => $memory_limit, '%memory_minimum_limit' => DRUPAL_MINIMUM_PHP_MEMORY_LIMIT]);
+      $description['phase'] = t('Depending on your configuration, Drupal can run with a %memory_limit PHP memory limit. However, a %memory_minimum_limit PHP memory limit or above is recommended, especially if your site uses additional custom or contributed modules.', ['%memory_limit' => $memory_limit, '%memory_minimum_limit' => Requirements::MINIMUM_PHP_MEMORY_LIMIT]);
     }
 
     if (!empty($description['phase'])) {
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index e38e8dd..d845b6e 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -10,6 +10,7 @@
 use Drupal\Core\Asset\AttachedAssetsInterface;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Queue\QueueGarbageCollectionInterface;
+use Drupal\Core\Requirements;
 use Drupal\Core\Database\Query\AlterableInterface;
 use Drupal\Core\Extension\Extension;
 use Drupal\Core\Extension\ExtensionDiscovery;
@@ -1033,7 +1034,7 @@ function _system_rebuild_module_data() {
     'description' => '',
     'package' => 'Other',
     'version' => NULL,
-    'php' => DRUPAL_MINIMUM_PHP,
+    'php' => Requirements::MINIMUM_PHP,
   ];
 
   // Read info files for each module.
diff --git a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
index 36ba1d6..780b33c 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
@@ -361,3 +361,22 @@ protected function systemListReset() {
   }
 
 }
+
+if (!defined('DRUPAL_EXTENSION_NAME_MAX_LENGTH')) {
+  define('DRUPAL_EXTENSION_NAME_MAX_LENGTH', 50);
+}
+if (!defined('DRUPAL_PHP_FUNCTION_PATTERN')) {
+  define('DRUPAL_PHP_FUNCTION_PATTERN', '[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*');
+}
+
+/**
+ * Minimum supported version of PHP.
+ *
+ * @deprecated in Drupal 8.6.0, will be removed before Drupal 9.0.0. Use
+ *   \Drupal\Core\Requirements::MINIMUM_PHP instead.
+ *
+ * @see https://www.drupal.org/node/2909361
+ */
+ if (!defined('DRUPAL_MINIMUM_PHP')) {
+   define('DRUPAL_MINIMUM_PHP', '5.3.10');
+ }