diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index f5a1a64..305016a 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         operating-system: [ ubuntu-latest ]
-        php-versions: ['8.0', '8.1', '8.2' ]
+        php-versions: ['8.1', '8.2', '8.3']
         
     runs-on: ${{ matrix.operating-system }}
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7a01445..fee6414 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,8 @@ The format is based on Keep a Changelog and this project adheres to Semantic Ver
 - `shared/config/lang.php` uses the 3.1 Lang format: `default` adapter (`file`, `deepl` or `google_translate`), `default_locale` and a section per remote adapter.
 - `save_remote_image()` uses the fluent `httpRequest()` helper.
 - CI runs `composer rector:check` after PHPStan.
+- The DTOs and services use `readonly` properties, and null values are cast to strings where they reach string functions (`trim()`, `strtotime()`, `preg_replace()`), as the Rector PHP 8.1 level set proposes.
+- **Breaking:** the minimum PHP version is 8.1 (`composer.json`, README, PHPStan `phpVersion` and the Rector level set), and CI runs on PHP 8.1, 8.2 and 8.3, following the framework (quantum-php/framework#595).
 
 ### Removed
 - The `Hook`, `Loader` and `libraries/` leftovers: the `hooks/` and `libraries/` folders, `shared/config/hooks.php` and the hooks and loader entries in the dependencies config.
diff --git a/README.md b/README.md
index c489ea4..3fcb952 100644
--- a/README.md
+++ b/README.md
@@ -32,7 +32,7 @@
 
 <h2>Installation</h2>
 
-<p><strong>Requirements:</strong> PHP 8.0 or newer.</p>
+<p><strong>Requirements:</strong> PHP 8.1 or newer.</p>
 
 <h3> 1. Create new project </h3>
 
diff --git a/composer.json b/composer.json
index d1c01f7..0d88aa5 100644
--- a/composer.json
+++ b/composer.json
@@ -25,7 +25,7 @@
     },
     "type": "project",
     "require": {
-        "php": "^8.0",
+        "php": "^8.1",
         "quantum/framework": "dev-master",
         "fakerphp/faker": "^1.15",
         "bluemmb/faker-picsum-photos-provider": "^2.0",
diff --git a/helpers/functions.php b/helpers/functions.php
index 2fa65bc..f4c5aea 100644
--- a/helpers/functions.php
+++ b/helpers/functions.php
@@ -30,7 +30,7 @@ function url_with_lang(string $lang): string
     }
 
     if (preg_match('/' . preg_quote(current_lang(), '/') . '/', route_uri())) {
-        return base_url() . preg_replace('/' . preg_quote(current_lang(), '/') . '/', $lang, route_uri(), 1);
+        return base_url() . preg_replace('/' . preg_quote(current_lang(), '/') . '/', (string) $lang, route_uri(), 1);
     }
 
     $url = base_url(true);
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index 4fc4a06..6870cdb 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -1,5 +1,5 @@
 parameters:
-    phpVersion: 80000
+    phpVersion: 80100
     level: 5
     paths:
         - shared
diff --git a/rector.php b/rector.php
index cacb402..6a2a654 100644
--- a/rector.php
+++ b/rector.php
@@ -20,7 +20,7 @@
         ClassPropertyAssignToConstructorPromotionRector::class,
     ])
     ->withSets([
-        LevelSetList::UP_TO_PHP_80,
+        LevelSetList::UP_TO_PHP_81,
         SetList::TYPE_DECLARATION,
     ])
     ->withRules([
diff --git a/shared/Commands/CommentCreateCommand.php b/shared/Commands/CommentCreateCommand.php
index 2959a98..7ab2e28 100644
--- a/shared/Commands/CommentCreateCommand.php
+++ b/shared/Commands/CommentCreateCommand.php
@@ -75,7 +75,7 @@ public function exec(): void
         $commentDto = new CommentDTO(
             $this->getArgument('post_uuid'),
             $this->getArgument('user_uuid'),
-            trim($this->getArgument('content'))
+            trim((string) $this->getArgument('content'))
         );
 
         service(CommentService::class)->addComment($commentDto);
diff --git a/shared/Commands/DemoCommand.php b/shared/Commands/DemoCommand.php
index 42c0ffb..f50d117 100644
--- a/shared/Commands/DemoCommand.php
+++ b/shared/Commands/DemoCommand.php
@@ -223,7 +223,7 @@ private function createModule(string $moduleName, string $template, bool $withAs
             ]);
         } catch (ProcessFailedException $e) {
             throw new RuntimeException(
-                trim($e->getProcess()->getOutput()) ?: $e->getMessage()
+                trim((string) $e->getProcess()->getOutput()) ?: $e->getMessage()
             );
         }
     }
diff --git a/shared/DTOs/CommentDTO.php b/shared/DTOs/CommentDTO.php
index edc50e2..e067a33 100644
--- a/shared/DTOs/CommentDTO.php
+++ b/shared/DTOs/CommentDTO.php
@@ -21,17 +21,17 @@ class CommentDTO
     /**
      * @var string
      */
-    private string $postUuid;
+    private readonly string $postUuid;
 
     /**
      * @var string
      */
-    private string $userUuid;
+    private readonly string $userUuid;
 
     /**
      * @var string
      */
-    private string $content;
+    private readonly string $content;
 
     /**
      * @param string $postUuid
diff --git a/shared/DTOs/PostDTO.php b/shared/DTOs/PostDTO.php
index cbe428c..17f4d7a 100644
--- a/shared/DTOs/PostDTO.php
+++ b/shared/DTOs/PostDTO.php
@@ -21,27 +21,27 @@ class PostDTO
     /**
      * @var string
      */
-    private string $title;
+    private readonly string $title;
 
     /**
      * @var string
      */
-    private string $content;
+    private readonly string $content;
 
     /**
      * @var string|null
      */
-    private ?string $userUuid;
+    private readonly ?string $userUuid;
 
     /**
      * @var string|null
      */
-    private ?string $image;
+    private readonly ?string $image;
 
     /**
      * @var string|null
      */
-    private ?string $uuid;
+    private readonly ?string $uuid;
 
     /**
      * @param string $title
diff --git a/shared/DTOs/UserDTO.php b/shared/DTOs/UserDTO.php
index 7dd9df4..e5ba02d 100644
--- a/shared/DTOs/UserDTO.php
+++ b/shared/DTOs/UserDTO.php
@@ -21,37 +21,37 @@ class UserDTO
     /**
      * @var string
      */
-    private string $email;
+    private readonly string $email;
 
     /**
      * @var string
      */
-    private string $password;
+    private readonly string $password;
 
     /**
      * @var string
      */
-    private string $firstname;
+    private readonly string $firstname;
 
     /**
      * @var string
      */
-    private string $lastname;
+    private readonly string $lastname;
 
     /**
      * @var string
      */
-    private string $role;
+    private readonly string $role;
 
     /**
      * @var string|null
      */
-    private ?string $uuid;
+    private readonly ?string $uuid;
 
     /**
      * @var string
      */
-    private string $image;
+    private readonly string $image;
 
     /**
      * @param string $email
diff --git a/shared/Services/AuthService.php b/shared/Services/AuthService.php
index f4d04dc..25203c2 100644
--- a/shared/Services/AuthService.php
+++ b/shared/Services/AuthService.php
@@ -30,7 +30,7 @@ class AuthService extends Service implements AuthServiceInterface
     /**
      * @var User
      */
-    private User $model;
+    private readonly User $model;
 
     /**
      * @throws BaseException
diff --git a/shared/Services/CommentService.php b/shared/Services/CommentService.php
index 14415f1..74b7dcb 100644
--- a/shared/Services/CommentService.php
+++ b/shared/Services/CommentService.php
@@ -29,7 +29,7 @@ class CommentService extends Service
      * @var Comment
      */
     private $model;
-    private CommentTransformer $transformer;
+    private readonly CommentTransformer $transformer;
 
     /**
      * @param CommentTransformer $transformer
diff --git a/shared/Services/PostService.php b/shared/Services/PostService.php
index 26a1509..22ad0a5 100644
--- a/shared/Services/PostService.php
+++ b/shared/Services/PostService.php
@@ -43,7 +43,7 @@ class PostService extends Service
     /**
      * @var PostTransformer
      */
-    private PostTransformer $transformer;
+    private readonly PostTransformer $transformer;
 
     /**
      * @param PostTransformer $transformer
diff --git a/shared/Transformers/CommentTransformer.php b/shared/Transformers/CommentTransformer.php
index 88d3a5d..607f206 100644
--- a/shared/Transformers/CommentTransformer.php
+++ b/shared/Transformers/CommentTransformer.php
@@ -28,7 +28,7 @@ public function transform($item): array
                 'image' => $item->image,
             ],
             'content' => $item->content,
-            'date' => date('Y-m-d H:i', strtotime($item->created_at)),
+            'date' => date('Y-m-d H:i', strtotime((string) $item->created_at)),
         ];
     }
 }
diff --git a/shared/Transformers/PostTransformer.php b/shared/Transformers/PostTransformer.php
index 92f3de1..f624513 100644
--- a/shared/Transformers/PostTransformer.php
+++ b/shared/Transformers/PostTransformer.php
@@ -30,7 +30,7 @@ public function transform($item): array
             'title' => $item->title,
             'content' => markdown_to_html($item->content, true),
             'image' => $this->buildImagePath($item),
-            'date' => date('Y/m/d H:i', strtotime($item->updated_at)),
+            'date' => date('Y/m/d H:i', strtotime((string) $item->updated_at)),
             'author' => $item->firstname . ' ' . $item->lastname,
         ];
     }