diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 499f552..5dee0a0 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -25,7 +25,15 @@
  * @todo Move this to an appropriate autoloadable class. See
  *   https://www.drupal.org/project/drupal/issues/2908079
  */
-const DRUPAL_MINIMUM_PHP = '5.5.9';
+const DRUPAL_MINIMUM_PHP = '7.0.0';
+
+/**
+ * Updates cannot be run on versions of PHP older than this version.
+ *
+ * @todo Move this to an appropriate autoloadable class. See
+ *   https://www.drupal.org/project/drupal/issues/2908079
+ */
+const DRUPAL_MINIMUM_UPDATE_PHP = '5.5.9';
 
 /**
  * Minimum recommended version of PHP.
diff --git a/core/includes/install.inc b/core/includes/install.inc
index 70c82b4..13f0372 100644
--- a/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -1116,7 +1116,7 @@ function install_profile_info($profile, $langcode = 'en') {
       'description' => '',
       'version' => NULL,
       'hidden' => FALSE,
-      'php' => DRUPAL_MINIMUM_PHP,
+      'php' => DRUPAL_MINIMUM_UPDATE_PHP,
       'config_install_path' => NULL,
     ];
     $profile_path = drupal_get_path('profile', $profile);
diff --git a/core/install.php b/core/install.php
index c88ffee..d367bce 100644
--- a/core/install.php
+++ b/core/install.php
@@ -25,8 +25,8 @@
 // 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) {
-  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.';
+if (version_compare(PHP_VERSION, '7.0.0') < 0) {
+  print 'Your PHP installation is too old. Drupal requires at least PHP 7.0.0. 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/ModuleExtensionList.php b/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
index c8f492b..07a1d83 100644
--- a/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ModuleExtensionList.php
@@ -28,7 +28,7 @@ class ModuleExtensionList extends ExtensionList {
     'description' => '',
     'package' => 'Other',
     'version' => NULL,
-    'php' => DRUPAL_MINIMUM_PHP,
+    'php' => DRUPAL_MINIMUM_UPDATE_PHP,
   ];
 
   /**
diff --git a/core/lib/Drupal/Core/Extension/ProfileExtensionList.php b/core/lib/Drupal/Core/Extension/ProfileExtensionList.php
index 7c41516..bb054d0 100644
--- a/core/lib/Drupal/Core/Extension/ProfileExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ProfileExtensionList.php
@@ -22,7 +22,7 @@ class ProfileExtensionList extends ExtensionList {
     'description' => '',
     'package' => 'Other',
     'version' => NULL,
-    'php' => DRUPAL_MINIMUM_PHP,
+    'php' => DRUPAL_MINIMUM_UPDATE_PHP,
   ];
 
   /**
diff --git a/core/lib/Drupal/Core/Extension/ThemeEngineExtensionList.php b/core/lib/Drupal/Core/Extension/ThemeEngineExtensionList.php
index c4d39ef..aef0d4e 100644
--- a/core/lib/Drupal/Core/Extension/ThemeEngineExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ThemeEngineExtensionList.php
@@ -21,7 +21,7 @@ class ThemeEngineExtensionList extends ExtensionList {
     'description' => '',
     'package' => 'Other',
     'version' => NULL,
-    'php' => DRUPAL_MINIMUM_PHP,
+    'php' => DRUPAL_MINIMUM_UPDATE_PHP,
   ];
 
   /**
diff --git a/core/lib/Drupal/Core/Extension/ThemeExtensionList.php b/core/lib/Drupal/Core/Extension/ThemeExtensionList.php
index 07a2f3f..5468c3f 100644
--- a/core/lib/Drupal/Core/Extension/ThemeExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ThemeExtensionList.php
@@ -48,7 +48,7 @@ class ThemeExtensionList extends ExtensionList {
       'comment_user_verification',
     ],
     'screenshot' => 'screenshot.png',
-    'php' => DRUPAL_MINIMUM_PHP,
+    'php' => DRUPAL_MINIMUM_UPDATE_PHP,
     'libraries' => [],
     'libraries_extend' => [],
     'libraries_override' => [],
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index 960bb7d..1deb173 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -188,22 +188,14 @@ 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]);
-    $requirements['php']['severity'] = REQUIREMENT_ERROR;
+    $requirements['php']['severity'] = ($phase === 'update') ? REQUIREMENT_WARNING : REQUIREMENT_ERROR;
     // If PHP is old, it's not safe to continue with the requirements check.
     return $requirements;
   }
-  if ((version_compare($phpversion, DRUPAL_RECOMMENDED_PHP) < 0) && ($phase === 'install' || $phase === 'runtime')) {
-    // Warn if still on PHP 5. If at least PHP 7.0, relax from "warning" to
-    // "info", and show it at runtime only, to not scare users while installing.
-    if (version_compare($phpversion, '7.0') < 0) {
-      $requirements['php']['description'] = t('Drupal will drop support for this version on March 6, 2019. Upgrade to PHP version %recommended or higher to ensure your site can receive updates and remain secure. See <a href="http://php.net/supported-versions.php">PHP\'s version support documentation</a> and the <a href=":php_requirements">Drupal 8 PHP requirements handbook page</a> for more information.', ['%recommended' => DRUPAL_RECOMMENDED_PHP, ':php_requirements' => 'https://www.drupal.org/docs/8/system-requirements/php']);
-      $requirements['php']['severity'] = REQUIREMENT_WARNING;
-    }
-    else {
-      if ($phase === 'runtime') {
-        $requirements['php']['description'] = t('It is recommended to upgrade to PHP version %recommended or higher for the best ongoing support.  See <a href="http://php.net/supported-versions.php">PHP\'s version support documentation</a> and the <a href=":php_requirements">Drupal 8 PHP requirements handbook page</a> for more information.', ['%recommended' => DRUPAL_RECOMMENDED_PHP, ':php_requirements' => 'https://www.drupal.org/docs/8/system-requirements/php']);
-        $requirements['php']['severity'] = REQUIREMENT_INFO;
-      }
+  if (version_compare($phpversion, DRUPAL_RECOMMENDED_PHP) < 0) {
+    if ($phase === 'runtime') {
+      $requirements['php']['description'] = t('It is recommended to upgrade to PHP version %recommended or higher for the best ongoing support.  See <a href="http://php.net/supported-versions.php">PHP\'s version support documentation</a> and the <a href=":php_requirements">Drupal 8 PHP requirements handbook page</a> for more information.', ['%recommended' => DRUPAL_RECOMMENDED_PHP, ':php_requirements' => 'https://www.drupal.org/docs/8/system-requirements/php']);
+      $requirements['php']['severity'] = REQUIREMENT_INFO;
     }
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php b/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
index 9c5fd58..4514b70 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
@@ -260,5 +260,5 @@ class TestThemeEngineExtensionList extends ThemeEngineExtensionList {
 }
 
 if (!defined('DRUPAL_MINIMUM_PHP')) {
-  define('DRUPAL_MINIMUM_PHP', '5.5.9');
+  define('DRUPAL_MINIMUM_PHP', '7.0.0');
 }
diff --git a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
index 97a5e93..b990f2c 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
@@ -147,5 +147,5 @@ protected function themeRegistryRebuild() {
 }
 
 if (!defined('DRUPAL_MINIMUM_PHP')) {
-  define('DRUPAL_MINIMUM_PHP', '5.5.9');
+  define('DRUPAL_MINIMUM_PHP', '7.0.0');
 }