diff --git a/.php-cs-fixer.dist.php b/.php-cs-fixer.dist.php
index ace047fcf23..9439e3615e4 100644
--- a/.php-cs-fixer.dist.php
+++ b/.php-cs-fixer.dist.php
@@ -201,6 +201,7 @@
         'no_whitespace_in_blank_line' => true,
         'non_printable_character' => true,
         'normalize_index_brace' => true,
+        'nullable_type_declaration_for_default_null_value' => true,
         'object_operator_without_whitespace' => true,
         'octal_notation' => true,
         'operator_linebreak' => [
diff --git a/src/Framework/Constraint/Constraint.php b/src/Framework/Constraint/Constraint.php
index 7702281a4f8..c4f67cc56ea 100644
--- a/src/Framework/Constraint/Constraint.php
+++ b/src/Framework/Constraint/Constraint.php
@@ -78,7 +78,7 @@ protected function matches(mixed $other): bool
      *
      * @throws ExpectationFailedException
      */
-    protected function fail(mixed $other, string $description, ComparisonFailure $comparisonFailure = null): never
+    protected function fail(mixed $other, string $description, ?ComparisonFailure $comparisonFailure = null): never
     {
         $failureDescription = sprintf(
             'Failed asserting that %s.',
diff --git a/src/Framework/Constraint/JsonMatches.php b/src/Framework/Constraint/JsonMatches.php
index 0000c8ef5a7..dc3be3b4cef 100644
--- a/src/Framework/Constraint/JsonMatches.php
+++ b/src/Framework/Constraint/JsonMatches.php
@@ -68,7 +68,7 @@ protected function matches(mixed $other): bool
      * @throws ExpectationFailedException
      * @throws InvalidJsonException
      */
-    protected function fail(mixed $other, string $description, ComparisonFailure $comparisonFailure = null): never
+    protected function fail(mixed $other, string $description, ?ComparisonFailure $comparisonFailure = null): never
     {
         if ($comparisonFailure === null) {
             [$error, $recodedOther] = Json::canonicalize($other);
diff --git a/src/Framework/Exception/Exception.php b/src/Framework/Exception/Exception.php
index 01531f5ec57..087729fb11b 100644
--- a/src/Framework/Exception/Exception.php
+++ b/src/Framework/Exception/Exception.php
@@ -40,7 +40,7 @@ class Exception extends RuntimeException implements \PHPUnit\Exception
 {
     protected array $serializableTrace;
 
-    public function __construct(string $message = '', int $code = 0, Throwable $previous = null)
+    public function __construct(string $message = '', int $code = 0, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
 
diff --git a/src/Framework/Exception/ExpectationFailedException.php b/src/Framework/Exception/ExpectationFailedException.php
index c1a4c043fd4..6d2b15013a3 100644
--- a/src/Framework/Exception/ExpectationFailedException.php
+++ b/src/Framework/Exception/ExpectationFailedException.php
@@ -25,7 +25,7 @@ final class ExpectationFailedException extends AssertionFailedError
 {
     protected ?ComparisonFailure $comparisonFailure = null;
 
-    public function __construct(string $message, ComparisonFailure $comparisonFailure = null, Exception $previous = null)
+    public function __construct(string $message, ?ComparisonFailure $comparisonFailure = null, ?Exception $previous = null)
     {
         $this->comparisonFailure = $comparisonFailure;
 
diff --git a/src/Framework/MockObject/Generator/Generator.php b/src/Framework/MockObject/Generator/Generator.php
index 0cc671c25f8..593cb8872c3 100644
--- a/src/Framework/MockObject/Generator/Generator.php
+++ b/src/Framework/MockObject/Generator/Generator.php
@@ -108,7 +108,7 @@ final class Generator
      * @throws RuntimeException
      * @throws UnknownTypeException
      */
-    public function testDouble(string $type, bool $mockObject, bool $markAsMockObject, ?array $methods = [], array $arguments = [], string $mockClassName = '', bool $callOriginalConstructor = true, bool $callOriginalClone = true, bool $callAutoload = true, bool $cloneArguments = true, bool $callOriginalMethods = false, object $proxyTarget = null, bool $allowMockingUnknownTypes = true, bool $returnValueGeneration = true): MockObject|Stub
+    public function testDouble(string $type, bool $mockObject, bool $markAsMockObject, ?array $methods = [], array $arguments = [], string $mockClassName = '', bool $callOriginalConstructor = true, bool $callOriginalClone = true, bool $callAutoload = true, bool $cloneArguments = true, bool $callOriginalMethods = false, ?object $proxyTarget = null, bool $allowMockingUnknownTypes = true, bool $returnValueGeneration = true): MockObject|Stub
     {
         if ($type === Traversable::class) {
             $type = Iterator::class;
@@ -240,7 +240,7 @@ public function testDoubleForInterfaceIntersection(array $interfaces, bool $mock
      *
      * @deprecated https://github.com/sebastianbergmann/phpunit/issues/5241
      */
-    public function mockObjectForAbstractClass(string $originalClassName, array $arguments = [], string $mockClassName = '', bool $callOriginalConstructor = true, bool $callOriginalClone = true, bool $callAutoload = true, array $mockedMethods = null, bool $cloneArguments = true): MockObject
+    public function mockObjectForAbstractClass(string $originalClassName, array $arguments = [], string $mockClassName = '', bool $callOriginalConstructor = true, bool $callOriginalClone = true, bool $callAutoload = true, ?array $mockedMethods = null, bool $cloneArguments = true): MockObject
     {
         if (class_exists($originalClassName, $callAutoload) ||
             interface_exists($originalClassName, $callAutoload)) {
@@ -302,7 +302,7 @@ interface_exists($originalClassName, $callAutoload)) {
      *
      * @deprecated https://github.com/sebastianbergmann/phpunit/issues/5243
      */
-    public function mockObjectForTrait(string $traitName, array $arguments = [], string $mockClassName = '', bool $callOriginalConstructor = true, bool $callOriginalClone = true, bool $callAutoload = true, array $mockedMethods = null, bool $cloneArguments = true): MockObject
+    public function mockObjectForTrait(string $traitName, array $arguments = [], string $mockClassName = '', bool $callOriginalConstructor = true, bool $callOriginalClone = true, bool $callAutoload = true, ?array $mockedMethods = null, bool $cloneArguments = true): MockObject
     {
         if (!trait_exists($traitName, $callAutoload)) {
             throw new UnknownTraitException($traitName);
@@ -385,7 +385,7 @@ public function objectForTrait(string $traitName, string $traitClassName = '', b
      *
      * @see https://github.com/sebastianbergmann/phpunit/issues/5476
      */
-    public function generate(string $type, bool $mockObject, bool $markAsMockObject, array $methods = null, string $mockClassName = '', bool $callOriginalClone = true, bool $callAutoload = true, bool $cloneArguments = true, bool $callOriginalMethods = false): MockClass
+    public function generate(string $type, bool $mockObject, bool $markAsMockObject, ?array $methods = null, string $mockClassName = '', bool $callOriginalClone = true, bool $callAutoload = true, bool $cloneArguments = true, bool $callOriginalMethods = false): MockClass
     {
         if ($mockClassName !== '') {
             return $this->generateCodeForTestDoubleClass(
@@ -571,7 +571,7 @@ private function userDefinedInterfaceMethods(string $interfaceName): array
      * @throws ReflectionException
      * @throws RuntimeException
      */
-    private function getObject(MockType $mockClass, string $type = '', bool $callOriginalConstructor = false, array $arguments = [], bool $callOriginalMethods = false, object $proxyTarget = null, bool $returnValueGeneration = true): object
+    private function getObject(MockType $mockClass, string $type = '', bool $callOriginalConstructor = false, array $arguments = [], bool $callOriginalMethods = false, ?object $proxyTarget = null, bool $returnValueGeneration = true): object
     {
         $className = $mockClass->generate();
         $object    = $this->instantiate($className, $callOriginalConstructor, $arguments);
diff --git a/src/Runner/PhptTestCase.php b/src/Runner/PhptTestCase.php
index 4a6636df787..9ee6ed48471 100644
--- a/src/Runner/PhptTestCase.php
+++ b/src/Runner/PhptTestCase.php
@@ -84,7 +84,7 @@ final class PhptTestCase implements Reorderable, SelfDescribing, Test
      *
      * @throws Exception
      */
-    public function __construct(string $filename, AbstractPhpProcess $phpUtil = null)
+    public function __construct(string $filename, ?AbstractPhpProcess $phpUtil = null)
     {
         if (!is_file($filename)) {
             throw new FileDoesNotExistException($filename);
diff --git a/src/Util/PHP/AbstractPhpProcess.php b/src/Util/PHP/AbstractPhpProcess.php
index 1a7b5a9d7a3..221508057a6 100644
--- a/src/Util/PHP/AbstractPhpProcess.php
+++ b/src/Util/PHP/AbstractPhpProcess.php
@@ -155,7 +155,7 @@ public function runTestJob(string $job, Test $test, string $processResultFile):
     /**
      * Returns the command based into the configurations.
      */
-    public function getCommand(array $settings, string $file = null): string
+    public function getCommand(array $settings, ?string $file = null): string
     {
         $runtime = new Runtime;