diff -u b/core/lib/Drupal/Core/Requirements.php b/core/lib/Drupal/Core/Requirements.php
--- b/core/lib/Drupal/Core/Requirements.php
+++ b/core/lib/Drupal/Core/Requirements.php
@@ -8,7 +8,14 @@
 final class Requirements {
 
   /**
-   * Minimum supported version of PHP.
+   * Minimum allowed version of PHP.
+   *
+   * Below this version:
+   * - The installer cannot be run.
+   * - Updates cannot be run.
+   * - Modules and themes cannot be enabled.
+   * - If a site managed to bypass all of the above, then an error is shown in
+   *   the status report and various fatal errors occur on various pages.
    */
   const MINIMUM_PHP = '7.0.8';
 
diff -u b/core/lib/autoloader.php b/core/lib/autoloader.php
--- b/core/lib/autoloader.php
+++ b/core/lib/autoloader.php
@@ -8,7 +8,7 @@
 use Drupal\Core\Requirements;
 
 // Checking minimum PHP version to avoid fatal errors.
-if (version_compare(PHP_VERSION, Requirements::MINIMUM_PHP) < 0) {
+if (defined('MAINTENANCE_MODE') && MAINTENANCE_MODE === 'install' && version_compare(PHP_VERSION, Requirements::MINIMUM_PHP) < 0) {
   print 'Your PHP installation is too old. Drupal requires at least PHP ' . Requirements::MINIMUM_PHP . '. See the <a href="https://www.drupal.org/requirements">system requirements</a> page for more information.';
   exit;
 }