diff --git a/core/lib/Drupal/Core/Password/PhpPassword.php b/core/lib/Drupal/Core/Password/PhpPassword.php
index fd29c3a..1f65dd4 100644
--- a/core/lib/Drupal/Core/Password/PhpPassword.php
+++ b/core/lib/Drupal/Core/Password/PhpPassword.php
@@ -93,6 +93,14 @@ public function check($password, UserInterface $account) {
    * {@inheritdoc}
    */
   public function userNeedsNewHash(UserInterface $account) {
+    // The PHP 5.5 password_needs_rehash() will return TRUE in two cases:
+    // - The the password is Drupal 6 or 7 password and it has been rehashed
+    //   during the migration. In this case the rehashed legacy hash is prefixed
+    //   to indicate and old-Drupal hash and will not comply the expected
+    //   password_needs_rehash() format.
+    // - The parameters of hashing engine were changed. For example the
+    //   parameter 'password_hash_cost' (the hashing cost) has been increased in
+    //   core.services.yml.
     return password_needs_rehash($account->getPassword(), PASSWORD_DEFAULT, $this->getOptions());
   }