diff --git a/Build/php-cs-fixer/config.php b/Build/php-cs-fixer/config.php
index 57ebfa08..755b1ae4 100644
--- a/Build/php-cs-fixer/config.php
+++ b/Build/php-cs-fixer/config.php
@@ -56,6 +56,10 @@
         'no_unused_imports' => true,
         'no_useless_else' => true,
         'no_useless_nullsafe_operator' => true,
+        'nullable_type_declaration' => [
+            'syntax' => 'question_mark',
+        ],
+        'nullable_type_declaration_for_default_null_value' => true,
         'ordered_imports' => ['imports_order' => ['class', 'function', 'const'], 'sort_algorithm' => 'alpha'],
         'php_unit_construct' => ['assertions' => ['assertEquals', 'assertSame', 'assertNotEquals', 'assertNotSame']],
         'php_unit_mock_short_will_return' => true,
diff --git a/Classes/Composer/ComposerPackageManager.php b/Classes/Composer/ComposerPackageManager.php
index c06015af..467783b4 100644
--- a/Classes/Composer/ComposerPackageManager.php
+++ b/Classes/Composer/ComposerPackageManager.php
@@ -47,7 +47,7 @@ final class ComposerPackageManager
 
     private static string $publicPath = '';
 
-    private static PackageInfo|null $rootPackage = null;
+    private static ?PackageInfo $rootPackage = null;
 
     /**
      * @var array<string, PackageInfo>
diff --git a/Classes/Core/BaseTestCase.php b/Classes/Core/BaseTestCase.php
index 5ae2a0e4..37a9c44c 100644
--- a/Classes/Core/BaseTestCase.php
+++ b/Classes/Core/BaseTestCase.php
@@ -108,7 +108,7 @@ protected function tearDown(): void
      */
     protected function getAccessibleMock(
         string $originalClassName,
-        array|null $methods = [],
+        ?array $methods = [],
         array $arguments = [],
         string $mockClassName = '',
         bool $callOriginalConstructor = true,
diff --git a/Classes/Core/Functional/Framework/DataHandling/ActionService.php b/Classes/Core/Functional/Framework/DataHandling/ActionService.php
index 210d99b7..79f67354 100644
--- a/Classes/Core/Functional/Framework/DataHandling/ActionService.php
+++ b/Classes/Core/Functional/Framework/DataHandling/ActionService.php
@@ -111,7 +111,7 @@ public function createNewRecords(int $pageId, array $tableRecordData): array
      * modifyRecord('tt_content', 42, ['hidden' => '1']); // Modify a single record
      * modifyRecord('tt_content', 42, ['hidden' => '1'], ['tx_irre_table' => [4]]); // Modify a record and delete a child
      */
-    public function modifyRecord(string $tableName, int $uid, array $recordData, array $deleteTableRecordIds = null)
+    public function modifyRecord(string $tableName, int $uid, array $recordData, ?array $deleteTableRecordIds = null)
     {
         $dataMap = [
             $tableName => [
@@ -266,7 +266,7 @@ public function clearWorkspaceRecords(array $tableRecordIds)
      * Example:
      * copyRecord('tt_content', 42, 5, ['header' => 'Testing #1']);
      */
-    public function copyRecord(string $tableName, int $uid, int $pageId, array $recordData = null): array
+    public function copyRecord(string $tableName, int $uid, int $pageId, ?array $recordData = null): array
     {
         $commandMap = [
             $tableName => [
@@ -302,7 +302,7 @@ public function copyRecord(string $tableName, int $uid, int $pageId, array $reco
      * @param array $recordData Additional record data to change when moving.
      * @return array
      */
-    public function moveRecord(string $tableName, int $uid, int $targetUid, array $recordData = null): array
+    public function moveRecord(string $tableName, int $uid, int $targetUid, ?array $recordData = null): array
     {
         $commandMap = [
             $tableName => [
diff --git a/Classes/Core/Functional/Framework/DataHandling/Scenario/DataHandlerFactory.php b/Classes/Core/Functional/Framework/DataHandling/Scenario/DataHandlerFactory.php
index 3f86913c..04b8fb0d 100644
--- a/Classes/Core/Functional/Framework/DataHandling/Scenario/DataHandlerFactory.php
+++ b/Classes/Core/Functional/Framework/DataHandling/Scenario/DataHandlerFactory.php
@@ -110,8 +110,8 @@ public function getSuggestedIds(): array
      */
     private function processEntities(
         array $settings,
-        string $nodeId = null,
-        string $parentId = null
+        ?string $nodeId = null,
+        ?string $parentId = null
     ): void {
         foreach ($settings as $entityName => $entitySettings) {
             $entityConfiguration = $this->provideEntityConfiguration($entityName);
@@ -135,8 +135,8 @@ private function processEntities(
     private function processEntityItem(
         EntityConfiguration $entityConfiguration,
         array $itemSettings,
-        string $nodeId = null,
-        string $parentId = null
+        ?string $nodeId = null,
+        ?string $parentId = null
     ): void {
         $values = $this->processEntityValues(
             $entityConfiguration,
@@ -199,7 +199,7 @@ private function processLanguageVariantItem(
         EntityConfiguration $entityConfiguration,
         array $itemSettings,
         array $ancestorIds,
-        string $nodeId = null
+        ?string $nodeId = null
     ): void {
         $values = $this->processEntityValues(
             $entityConfiguration,
@@ -240,7 +240,7 @@ private function processVersionVariantItem(
         EntityConfiguration $entityConfiguration,
         array $itemSettings,
         string $ancestorId,
-        string $nodeId = null
+        ?string $nodeId = null
     ): void {
         if (isset($itemSettings['self'])) {
             throw new \LogicException(
@@ -284,8 +284,8 @@ private function processVersionVariantItem(
     private function processEntityValues(
         EntityConfiguration $entityConfiguration,
         array $itemSettings,
-        string $nodeId = null,
-        string $parentId = null
+        ?string $nodeId = null,
+        ?string $parentId = null
     ): array {
         if (isset($itemSettings['self']) && isset($itemSettings['version'])) {
             throw new \LogicException(
diff --git a/Classes/Core/Functional/Framework/Frontend/Collector.php b/Classes/Core/Functional/Framework/Frontend/Collector.php
index 135e8bcb..bd5465b4 100644
--- a/Classes/Core/Functional/Framework/Frontend/Collector.php
+++ b/Classes/Core/Functional/Framework/Frontend/Collector.php
@@ -44,7 +44,7 @@ public function setContentObjectRenderer(ContentObjectRenderer $cObj): void
         $this->cObj = $cObj;
     }
 
-    public function addRecordData($content, array $configuration = null, ServerRequestInterface $request): void
+    public function addRecordData($content, ?array $configuration = null, ?ServerRequestInterface $request = null): void
     {
         $recordIdentifier = $this->cObj->currentRecord;
         [$tableName] = explode(':', $recordIdentifier);
@@ -63,7 +63,7 @@ public function addRecordData($content, array $configuration = null, ServerReque
         }
     }
 
-    public function addFileData($content, array $configuration = null, ServerRequestInterface $request): void
+    public function addFileData($content, ?array $configuration = null, ?ServerRequestInterface $request = null): void
     {
         $currentFile = $this->cObj->getCurrentFile();
 
@@ -84,7 +84,7 @@ public function addFileData($content, array $configuration = null, ServerRequest
         $this->addToStructure($levelIdentifier, $recordIdentifier, $recordData);
     }
 
-    public function attachSection(string $content, array $configuration = null): void
+    public function attachSection(string $content, ?array $configuration = null): void
     {
         $section = [
             'structure' => $this->structure,
diff --git a/Classes/Core/Functional/Framework/Frontend/Renderer.php b/Classes/Core/Functional/Framework/Frontend/Renderer.php
index e0870daa..5468576a 100644
--- a/Classes/Core/Functional/Framework/Frontend/Renderer.php
+++ b/Classes/Core/Functional/Framework/Frontend/Renderer.php
@@ -40,7 +40,7 @@ class Renderer implements SingletonInterface
      * @param string $content
      * @param array|null $configuration
      */
-    public function parseValues($content, array $configuration = null)
+    public function parseValues($content, ?array $configuration = null)
     {
         if (empty($content)) {
             return;
@@ -84,7 +84,7 @@ public function parseValues($content, array $configuration = null)
      * @param string $content
      * @param array|null $configuration
      */
-    public function renderValues($content, array $configuration = null)
+    public function renderValues($content, ?array $configuration = null)
     {
         if (empty($configuration['values.'])) {
             return;
@@ -112,7 +112,7 @@ public function addSection(array $section, $as = null)
      * @param array|null $configuration
      * @return string
      */
-    public function renderSections($content, array $configuration = null)
+    public function renderSections($content, ?array $configuration = null)
     {
         return json_encode($this->sections);
     }
diff --git a/Classes/Core/Functional/Framework/Frontend/ResponseContent.php b/Classes/Core/Functional/Framework/Frontend/ResponseContent.php
index ad66cc96..a921df46 100644
--- a/Classes/Core/Functional/Framework/Frontend/ResponseContent.php
+++ b/Classes/Core/Functional/Framework/Frontend/ResponseContent.php
@@ -47,7 +47,7 @@ class ResponseContent
 
     final public function __construct() {}
 
-    public static function fromString(string $data, ResponseContent $target = null): ResponseContent
+    public static function fromString(string $data, ?ResponseContent $target = null): ResponseContent
     {
         $target = $target ?? new static();
         $content = json_decode($data, true);
diff --git a/Classes/Core/Functional/FunctionalTestCase.php b/Classes/Core/Functional/FunctionalTestCase.php
index 58edd8e2..45ca7044 100644
--- a/Classes/Core/Functional/FunctionalTestCase.php
+++ b/Classes/Core/Functional/FunctionalTestCase.php
@@ -898,7 +898,7 @@ protected function addTypoScriptToTemplateRecord(int $pageId, string $typoScript
      */
     protected function executeFrontendSubRequest(
         InternalRequest $request,
-        InternalRequestContext $context = null,
+        ?InternalRequestContext $context = null,
         bool $followRedirects = false
     ): ResponseInterface {
         if ($context === null) {
diff --git a/Classes/Core/PackageCollection.php b/Classes/Core/PackageCollection.php
index 6a3ff288..cbef267e 100644
--- a/Classes/Core/PackageCollection.php
+++ b/Classes/Core/PackageCollection.php
@@ -82,7 +82,7 @@ public function getPackages(): array
         return $this->packages;
     }
 
-    public function sortPackages(DependencyOrderingService $dependencyOrderingService = null): void
+    public function sortPackages(?DependencyOrderingService $dependencyOrderingService = null): void
     {
         $sortedPackageKeys = $this->resolveSortedPackageKeys($dependencyOrderingService);
         usort(
@@ -97,7 +97,7 @@ public function sortPackages(DependencyOrderingService $dependencyOrderingServic
      * @param array<PackageKey, StateConfiguration> $packageStates
      * @return array<PackageKey, StateConfiguration>
      */
-    public function sortPackageStates(array $packageStates, DependencyOrderingService $dependencyOrderingService = null): array
+    public function sortPackageStates(array $packageStates, ?DependencyOrderingService $dependencyOrderingService = null): array
     {
         $sortedPackageKeys = $this->resolveSortedPackageKeys($dependencyOrderingService);
         uksort(
@@ -117,7 +117,7 @@ public function sortPackageStates(array $packageStates, DependencyOrderingServic
      *
      * @return list<PackageKey>
      */
-    public function resolveSortedPackageKeys(DependencyOrderingService $dependencyOrderingService = null): array
+    public function resolveSortedPackageKeys(?DependencyOrderingService $dependencyOrderingService = null): array
     {
         $dependencyOrderingService ??= GeneralUtility::makeInstance(DependencyOrderingService::class);
         $allPackageConstraints = $this->resolveAllPackageConstraints();