diff --git a/src/FluentDOM.php b/src/FluentDOM.php
index 7f06bc43..6b5166d3 100644
--- a/src/FluentDOM.php
+++ b/src/FluentDOM.php
@@ -111,7 +111,7 @@ public static function QueryCss(
    *
    * @throws InvalidArgument
    */
-  public static function setLoader(Loadable $loader = NULL): void {
+  public static function setLoader(?Loadable $loader = NULL): void {
     if (!$loader) {
       self::$_loader = NULL;
       return;
diff --git a/src/FluentDOM/Creator.php b/src/FluentDOM/Creator.php
index ee4c0880..e593fb4e 100644
--- a/src/FluentDOM/Creator.php
+++ b/src/FluentDOM/Creator.php
@@ -108,7 +108,7 @@ public function pi(string $target, string $content): DOM\ProcessingInstruction {
       return $this->_document->createProcessingInstruction($target, $content);
     }
 
-    public function each(iterable $traversable, callable $map = NULL): Appendable {
+    public function each(iterable $traversable, ?callable $map = NULL): Appendable {
       return new Creator\Nodes($traversable, $map);
     }
   }
diff --git a/src/FluentDOM/Creator/Nodes.php b/src/FluentDOM/Creator/Nodes.php
index 6a1f19fb..530e2ccb 100644
--- a/src/FluentDOM/Creator/Nodes.php
+++ b/src/FluentDOM/Creator/Nodes.php
@@ -27,7 +27,7 @@ class Nodes implements Appendable, \OuterIterator {
 
     private ?\Iterator $_iterator = NULL;
 
-    public function __construct(iterable $iterable, callable $map = NULL) {
+    public function __construct(iterable $iterable, ?callable $map = NULL) {
       $this->_iterable = $iterable;
       $this->_map = Constraints::filterCallable($map);
     }
diff --git a/src/FluentDOM/DOM/Document.php b/src/FluentDOM/DOM/Document.php
index e63d20a4..d84fee39 100644
--- a/src/FluentDOM/DOM/Document.php
+++ b/src/FluentDOM/DOM/Document.php
@@ -107,7 +107,7 @@ public function registerNamespace(string $prefix, string $namespaceURI): void {
      *
      * @throws \LogicException
      */
-    public function namespaces(iterable $namespaces = NULL): Namespaces {
+    public function namespaces(?iterable $namespaces = NULL): Namespaces {
       if (NULL !== $namespaces) {
         $this->_namespaces->assign([]);
         foreach($namespaces as $prefix => $namespaceURI) {
@@ -130,8 +130,8 @@ public function namespaces(iterable $namespaces = NULL): Namespaces {
      */
     public function createElement(
       string $name,
-      string|array $value = NULL,
-      array $attributes = NULL
+      string|array|null $value = NULL,
+      ?array $attributes = NULL
     ): Element {
       [$prefix, $localName] = QualifiedName::split($name);
       $namespaceURI = '';
@@ -164,7 +164,7 @@ public function createElement(
     public function createElementNS(
       string|null $namespace,
       string $qualifiedName,
-      string $value = NULL
+      ?string $value = NULL
     ): Element {
       /** @var Element $node */
       $node = parent::createElementNS($namespace, $qualifiedName);
@@ -180,7 +180,7 @@ public function createElementNS(
      *
      * @throws \LogicException|\DOMException
      */
-    public function createAttribute(string $name, string $value = NULL): Attribute {
+    public function createAttribute(string $name, ?string $value = NULL): Attribute {
       [$prefix] = QualifiedName::split($name);
       if (empty($prefix)) {
         $node = parent::createAttribute($name);
@@ -201,7 +201,7 @@ public function createAttribute(string $name, string $value = NULL): Attribute {
     public function appendElement(
       string $name,
       string|array $value = '',
-      array $attributes = NULL
+      ?array $attributes = NULL
     ): Element {
       $this->appendChild(
         $node = $this->createElement($name, $value, $attributes)
@@ -211,8 +211,8 @@ public function appendElement(
 
     private function appendAttributes(
       \DOMElement $node,
-      string|array $content = NULL,
-      array $attributes = NULL
+      string|array|null $content = NULL,
+      ?array $attributes = NULL
     ): void {
       if (\is_array($content)) {
         /** @noinspection CallableParameterUseCaseInTypeContextInspection */
@@ -225,7 +225,7 @@ private function appendAttributes(
       }
     }
 
-    private function appendContent(\DOMElement $node, string|array $content = NULL): void {
+    private function appendContent(\DOMElement $node, string|array|null $content = NULL): void {
       if (!((empty($content) && !\is_numeric($content)) || \is_array($content) )) {
         $node->appendChild($this->createTextNode((string)$content));
       }
@@ -237,7 +237,7 @@ private function appendContent(\DOMElement $node, string|array $content = NULL):
      * Overloading saveXML() with a removed type hint triggers an E_STRICT error,
      * so the function needs a new name. :-(
      */
-    public function saveXML(\DOMNode|\DOMNodeList $context = NULL, int $options = NULL): string {
+    public function saveXML(\DOMNode|\DOMNodeList|null $context = NULL, ?int $options = NULL): string {
       if ($context instanceof \DOMNodeList) {
         $result = '';
         foreach ($context as $node) {
@@ -251,7 +251,7 @@ public function saveXML(\DOMNode|\DOMNodeList $context = NULL, int $options = NU
     /**
      * @deprecated
      */
-    public function toXml(\DOMNode|\DOMNodeList $context = NULL, int $options = NULL): string {
+    public function toXml(\DOMNode|\DOMNodeList|null $context = NULL, ?int $options = NULL): string {
       return $this->saveXML($context, $options);
     }
       /**
@@ -269,14 +269,14 @@ public function __toString(): string {
      *
      * @deprecated
      */
-    public function toHtml(\DOMNode|\DOMNodeList $context = NULL): string {
+    public function toHtml(\DOMNode|\DOMNodeList|null $context = NULL): string {
       return $this->saveHTML($context);
     }
 
     /**
      * Allow to save HTML fragments, providing a node list
      */
-    public function saveHTML(\DOMNode|\DOMNodeList $context = NULL): string {
+    public function saveHTML(\DOMNode|\DOMNodeList|null $context = NULL): string {
       if ($context instanceof \DOMDocumentFragment) {
         $context = $context->childNodes;
       }
@@ -323,7 +323,7 @@ public function getElementsByTagName(string $qualifiedName): \DOMNodeList {
      * @throws \DOMException
      */
     public function createDocumentType(
-      string $qualifiedName = NULL, string $publicId = NULL, string $systemId = NULL
+      ?string $qualifiedName = NULL, ?string $publicId = NULL, ?string $systemId = NULL
     ): \DOMDocumentType {
       return (new Implementation())->createDocumentType($qualifiedName, (string)$publicId, (string)$systemId);
     }
diff --git a/src/FluentDOM/DOM/DocumentFragment.php b/src/FluentDOM/DOM/DocumentFragment.php
index 310736dc..8d510709 100644
--- a/src/FluentDOM/DOM/DocumentFragment.php
+++ b/src/FluentDOM/DOM/DocumentFragment.php
@@ -64,7 +64,7 @@ public function getIterator(): \Iterator {
      *
      * @throws \InvalidArgumentException
      */
-    public function namespaces(iterable|\DOMElement $namespaces = NULL): Namespaces {
+    public function namespaces(iterable|\DOMElement|null $namespaces = NULL): Namespaces {
       if (NULL !== $namespaces || (!$this->_namespaces instanceof Namespaces)) {
         $this->_namespaces = new Namespaces();
       }
@@ -110,7 +110,7 @@ public function registerNamespace(string $prefix, string $namespaceURI): void {
      * @return bool
      * @throws \InvalidArgumentException
      */
-    public function appendXml(string $data, iterable|\DOMElement $namespaces = NULL): bool {
+    public function appendXml(string $data, null|iterable|\DOMElement $namespaces = NULL): bool {
       $namespaces = $this->namespaces($namespaces);
       if (\count($namespaces) === 0) {
         return parent::appendXML($data);
@@ -138,7 +138,7 @@ public function appendXml(string $data, iterable|\DOMElement $namespaces = NULL)
      * @throws \LogicException|\DOMException
      */
     public function appendElement(
-      string $name, string|array $content = '', array $attributes = NULL
+      string $name, string|array $content = '', ?array $attributes = NULL
     ): Element {
       $this->appendChild(
         $node = $this->ownerDocument->createElement($name, $content, $attributes)
diff --git a/src/FluentDOM/DOM/Element.php b/src/FluentDOM/DOM/Element.php
index 76033ce0..0dd85d6b 100644
--- a/src/FluentDOM/DOM/Element.php
+++ b/src/FluentDOM/DOM/Element.php
@@ -218,7 +218,7 @@ public function append(...$nodes): void {
      * @throws \LogicException|\DOMException
      */
     public function appendElement(
-      string $qualifiedName, string|array $content = '', array $attributes = NULL
+      string $qualifiedName, string|array $content = '', ?array $attributes = NULL
     ): Element {
       $this->appendChild(
         $node = $this->getDocument()->createElement($qualifiedName, $content, $attributes)
@@ -452,7 +452,7 @@ private function getDocument(): Document {
      *
      * @throws \LogicException
      */
-    public function applyNamespaces(string|array $prefixes = NULL): void {
+    public function applyNamespaces(string|array|null $prefixes = NULL): void {
       if ($prefixes !== NULL && !\is_array($prefixes)) {
         $prefixes = [$prefixes];
       }
diff --git a/src/FluentDOM/DOM/Implementation.php b/src/FluentDOM/DOM/Implementation.php
index f46e4464..4a44288c 100644
--- a/src/FluentDOM/DOM/Implementation.php
+++ b/src/FluentDOM/DOM/Implementation.php
@@ -21,9 +21,9 @@
   class Implementation extends \DOMImplementation {
 
     public function createDocument(
-      string $namespace = NULL,
-      string $qualifiedName = NULL,
-      DOMDocumentType $doctype = NULL
+      ?string $namespace = NULL,
+      ?string $qualifiedName = NULL,
+      ?DOMDocumentType $doctype = NULL
     ): Document {
       $document = new Document();
       if ($doctype) {
diff --git a/src/FluentDOM/DOM/Node/ParentNode/Implementation.php b/src/FluentDOM/DOM/Node/ParentNode/Implementation.php
index 748981ed..92205801 100644
--- a/src/FluentDOM/DOM/Node/ParentNode/Implementation.php
+++ b/src/FluentDOM/DOM/Node/ParentNode/Implementation.php
@@ -25,7 +25,7 @@
    */
   trait Implementation {
 
-    abstract public function insertBefore(\DOMNode $newChild, \DOMNode $refChild = NULL);
+    abstract public function insertBefore(\DOMNode $newChild, ?\DOMNode $refChild = NULL);
 
     abstract public function appendChild(\DOMNode $newChild);
 
diff --git a/src/FluentDOM/DOM/Node/WholeText.php b/src/FluentDOM/DOM/Node/WholeText.php
index 6897a553..3a072a9c 100644
--- a/src/FluentDOM/DOM/Node/WholeText.php
+++ b/src/FluentDOM/DOM/Node/WholeText.php
@@ -50,7 +50,7 @@ public function replaceWholeText($content): static|NULL|Text|CdataSection {
         }
         return TRUE;
       };
-      $replaceNode = static function(\DOMNode $node = NULL) use ($canReplaceEntity) {
+      $replaceNode = static function(?\DOMNode $node = NULL) use ($canReplaceEntity) {
         if (
           $node instanceof \DOMNode &&
           !(
diff --git a/src/FluentDOM/DOM/Node/Xpath.php b/src/FluentDOM/DOM/Node/Xpath.php
index 7cca5a1a..9f0c3336 100644
--- a/src/FluentDOM/DOM/Node/Xpath.php
+++ b/src/FluentDOM/DOM/Node/Xpath.php
@@ -28,7 +28,7 @@ trait Xpath {
      * element.
      */
     public function evaluate(
-      string $expression, Node $context = NULL
+      string $expression, ?Node $context = NULL
     ): string|float|bool|\DOMNodeList {
       $document = $this instanceof Document
         ? $this
diff --git a/src/FluentDOM/DOM/Xpath.php b/src/FluentDOM/DOM/Xpath.php
index e3c9bda0..45051588 100644
--- a/src/FluentDOM/DOM/Xpath.php
+++ b/src/FluentDOM/DOM/Xpath.php
@@ -59,7 +59,7 @@ public function registerNamespace(string $prefix, string $namespace): bool {
      * is disabled by default.
      */
     public function evaluate(
-      string $expression, \DOMNode $contextNode = NULL, $registerNodeNS = NULL
+      string $expression, ?\DOMNode $contextNode = NULL, $registerNodeNS = NULL
     ): string|float|bool|\DOMNodeList {
       $registerNodeNS = $registerNodeNS ?? $this->registerNodeNamespaces;
       return parent::evaluate($expression, $contextNode, (bool)$registerNodeNS);
@@ -69,7 +69,7 @@ public function evaluate(
      * Fetch nodes or scalar values from the DOM using Xpath expression.
      */
     public function __invoke(
-      string $expression, \DOMNode $contextNode = NULL
+      string $expression, ?\DOMNode $contextNode = NULL
     ): string|float|bool|\DOMNodeList {
       return $this->evaluate($expression, $contextNode);
     }
@@ -83,7 +83,7 @@ public function __invoke(
      * @deprecated
      */
     public function query(
-      string $expression, \DOMNode $contextNode = NULL, bool $registerNodeNS = NULL
+      string $expression, ?\DOMNode $contextNode = NULL, ?bool $registerNodeNS = NULL
     ): ?\DOMNodeList {
       $result = $this->evaluate($expression, $contextNode, $registerNodeNS);
       return $result instanceof \DOMNodeList ? $result : NULL;
@@ -97,7 +97,7 @@ public function query(
      * @param NULL|bool $registerNodeNS
      * @return \DOMNode|NULL
      */
-    public function firstOf(string $expression, \DOMNode $contextNode = NULL, bool $registerNodeNS = NULL): ?\DOMNode {
+    public function firstOf(string $expression, ?\DOMNode $contextNode = NULL, ?bool $registerNodeNS = NULL): ?\DOMNode {
       $nodes = $this->evaluate($expression, $contextNode, $registerNodeNS);
       if ($nodes instanceof \DOMNodeList && $nodes->length > 0) {
         return $nodes->item(0);
diff --git a/src/FluentDOM/Loader/HtmlLoader.php b/src/FluentDOM/Loader/HtmlLoader.php
index 4120cceb..b9c366b7 100644
--- a/src/FluentDOM/Loader/HtmlLoader.php
+++ b/src/FluentDOM/Loader/HtmlLoader.php
@@ -73,7 +73,7 @@ function() use ($source, $contentType, $options) {
       return NULL;
     }
 
-    private function ensureEncodingPI(string $source, string $encoding = NULL, bool $force = FALSE): string {
+    private function ensureEncodingPI(string $source, ?string $encoding = NULL, bool $force = FALSE): string {
       $hasXmlPi = \preg_match('(<\\?xml\\s)', $source);
       if (!$force && ($charset = $this->getCharsetFromMetaTag($source))) {
         $encoding = (string)$charset;
diff --git a/src/FluentDOM/Loader/Json/JsonDOMLoader.php b/src/FluentDOM/Loader/Json/JsonDOMLoader.php
index 03de5e8b..45e5c784 100644
--- a/src/FluentDOM/Loader/Json/JsonDOMLoader.php
+++ b/src/FluentDOM/Loader/Json/JsonDOMLoader.php
@@ -118,7 +118,7 @@ private function prepareOnMapKey($options): ?callable {
      *
      * function callback(string $key, bool $isArrayElement) {}
      */
-    public function onMapKey(callable $callback = NULL): ?callable {
+    public function onMapKey(?callable $callback = NULL): ?callable {
       if (NULL !== $callback) {
         $this->_onMapKey = Constraints::filterCallable($callback);
       }
diff --git a/src/FluentDOM/Loader/LoaderResult.php b/src/FluentDOM/Loader/LoaderResult.php
index d667e7a0..aa8c72ef 100644
--- a/src/FluentDOM/Loader/LoaderResult.php
+++ b/src/FluentDOM/Loader/LoaderResult.php
@@ -20,7 +20,7 @@ class LoaderResult {
     private NULL|\DOMNode|iterable $_selection;
 
     public function __construct(
-      Document $document, string $contentType, \DOMNode|iterable $selection = NULL
+      Document $document, string $contentType, null|\DOMNode|iterable $selection = NULL
     ) {
       $this->_document = $document;
       $this->_contentType = $contentType;
diff --git a/src/FluentDOM/Loader/Text/CSVLoader.php b/src/FluentDOM/Loader/Text/CSVLoader.php
index 36852c2a..91d7f226 100644
--- a/src/FluentDOM/Loader/Text/CSVLoader.php
+++ b/src/FluentDOM/Loader/Text/CSVLoader.php
@@ -91,7 +91,7 @@ public function loadFragment($source, string $contentType, $options = []): ?Docu
      * @throws \DOMException|UnattachedNode
      */
     private function appendLines(
-      \DOMNode $parent, iterable $lines, bool $hasHeaderLine, array $columns = NULL
+      \DOMNode $parent, iterable $lines, bool $hasHeaderLine, ?array $columns = NULL
     ): void {
       $document = Implementation::getNodeDocument($parent);
       $headers = NULL;
@@ -126,7 +126,7 @@ private function appendField(Element $parent, string $name, string $value): void
       }
     }
 
-    private function getHeaders(array $record, bool $hasHeaderLine, array $columns = NULL): array {
+    private function getHeaders(array $record, bool $hasHeaderLine, ?array $columns = NULL): array {
       if (\is_array($columns)) {
         $headers = [];
         foreach ($record as $index => $field) {
diff --git a/src/FluentDOM/Loader/XDM/JsonAsXDMLoader.php b/src/FluentDOM/Loader/XDM/JsonAsXDMLoader.php
index 012114a2..83d0f5d1 100644
--- a/src/FluentDOM/Loader/XDM/JsonAsXDMLoader.php
+++ b/src/FluentDOM/Loader/XDM/JsonAsXDMLoader.php
@@ -101,7 +101,7 @@ public function loadFragment(
      * @throws UnattachedNode|\DOMException
      */
     protected function transferTo(
-      \DOMNode $target, mixed $json, string $key = NULL, int $recursions = 100
+      \DOMNode $target, mixed $json, ?string $key = NULL, int $recursions = 100
     ): void {
       if ($recursions < 1) {
         return;
diff --git a/src/FluentDOM/Loaders.php b/src/FluentDOM/Loaders.php
index 665be036..eaa84fcc 100644
--- a/src/FluentDOM/Loaders.php
+++ b/src/FluentDOM/Loaders.php
@@ -29,7 +29,7 @@ class Loaders implements \IteratorAggregate, Loadable {
     /**
      * Store the a list of loaders if provided.
      */
-    public function __construct(iterable $list = NULL) {
+    public function __construct(?iterable $list = NULL) {
       if (is_iterable($list)) {
         /** @var array|\Traversable $list */
         foreach ($list as $loader) {
diff --git a/src/FluentDOM/Nodes.php b/src/FluentDOM/Nodes.php
index 5245836f..4575d1d2 100644
--- a/src/FluentDOM/Nodes.php
+++ b/src/FluentDOM/Nodes.php
@@ -77,7 +77,7 @@ class Nodes implements \ArrayAccess, \Countable, \IteratorAggregate {
      * @throws \OutOfBoundsException
      * @throws \LogicException
      */
-    public function __construct(mixed $source = NULL, string $contentType = NULL) {
+    public function __construct(mixed $source = NULL, ?string $contentType = NULL) {
       if (NULL !== $source) {
         $this->load($source, $contentType);
       } elseif (NULL !== $contentType) {
@@ -96,7 +96,7 @@ public function __construct(mixed $source = NULL, string $contentType = NULL) {
      */
     public function load(
       mixed $source,
-      string $contentType = NULL,
+      ?string $contentType = NULL,
       iterable $options = []
     ): self {
       $contentType = $contentType ?: 'text/xml';
@@ -155,7 +155,7 @@ private function prepareSource(
      * Set the loaders list.
      * @throws \InvalidArgumentException
      */
-    public function loaders(Loadable|iterable $loaders = NULL): Loadable {
+    public function loaders(Loadable|iterable|null $loaders = NULL): Loadable {
       if (NULL !== $loaders) {
         if ($loaders instanceof Loadable) {
           $this->_loaders = $loaders;
@@ -199,8 +199,8 @@ public function item(int $position): \DOMNode|NULL {
      * @throws \LogicException
      */
     public function xpath(
-      string $expression = NULL,
-      \DOMNode $contextNode = NULL
+      ?string $expression = NULL,
+      ?\DOMNode $contextNode = NULL
     ): Xpath|\DOMNodeList|float|string {
       if (NULL !== $expression) {
         return $this->getXpath()->evaluate($expression, $contextNode);
@@ -264,7 +264,7 @@ private function applyNamespaces(): void {
      *
      * @throws \LogicException
      */
-    public function formatOutput(string $contentType = NULL): self {
+    public function formatOutput(?string $contentType = NULL): self {
       if (NULL !== $contentType) {
         $this->setContentType($contentType);
       }
@@ -358,7 +358,7 @@ public function prepareSelector(string $selector, int $contextMode): string {
      * @throws \InvalidArgumentException
      */
     public function getSelectorCallback(
-      callable|string|iterable|\DOMNode $selector = NULL
+      callable|string|iterable|\DOMNode|null $selector = NULL
     ): ?\Closure {
       if (NULL === $selector) {
         return NULL;
@@ -395,7 +395,7 @@ public function getSelectorCallback(
     /**
      * Test that selector matches context and return TRUE/FALSE
      */
-    protected function matches(string $selector, \DOMNode $context = NULL): bool {
+    protected function matches(string $selector, ?\DOMNode $context = NULL): bool {
       $check = $this->xpath->evaluate(
         $this->prepareSelector($selector, self::CONTEXT_SELF), $context
       );
@@ -576,7 +576,7 @@ public function __toString(): string {
      * @throws \OutOfBoundsException
      * @throws \InvalidArgumentException
      */
-    public function spawn(iterable|\DOMElement|\DOMCharacterData $elements = NULL): static {
+    public function spawn(iterable|\DOMElement|\DOMCharacterData|null $elements = NULL): static {
       $result = clone $this;
       $result->_parent = $this;
       $result->_document = $this->getDocument();
@@ -606,7 +606,7 @@ public function end(): self {
      * @throws \OutOfBoundsException
      */
     public function push(
-      iterable|\DOMElement|\DOMCharacterData $elements = NULL, bool $ignoreTextNodes = FALSE
+      iterable|\DOMElement|\DOMCharacterData|null $elements = NULL, bool $ignoreTextNodes = FALSE
     ): static {
       if (Constraints::filterNode($elements, $ignoreTextNodes)) {
         if ($elements->ownerDocument !== $this->_document) {
@@ -641,7 +641,7 @@ public function push(
      */
     public function each(
       callable $function,
-      callable|bool $elementsFilter = NULL
+      callable|bool|null $elementsFilter = NULL
     ): self {
       if (TRUE === $elementsFilter) {
         $filter = static function ($node) {
@@ -747,7 +747,7 @@ private function prepareSelectorAsFilter(string $selector, int $contextMode): \C
      * @throws \LogicException
      * @throws \InvalidArgumentException
      */
-    public function index(string|\DOMNode|iterable $selector = NULL): int {
+    public function index(string|\DOMNode|iterable|null $selector = NULL): int {
       if (\count($this->_nodes) > 0) {
         if (NULL === $selector) {
           return (int)$this->xpath(
@@ -810,7 +810,7 @@ public function unique(array $array): array {
     }
 
     public function serializerFactories(
-      Serializer\Factory\Group $factories = NULL
+      ?Serializer\Factory\Group $factories = NULL
     ): Serializer\Factory\Group {
       if (NULL !== $factories) {
         $this->_serializerFactories = $factories;
diff --git a/src/FluentDOM/Nodes/Builder.php b/src/FluentDOM/Nodes/Builder.php
index 5735c597..d4e5fc14 100644
--- a/src/FluentDOM/Nodes/Builder.php
+++ b/src/FluentDOM/Nodes/Builder.php
@@ -61,7 +61,7 @@ private function getNodeList(
      * @throws \InvalidArgumentException
      * @throws \LogicException
      */
-    public function getTargetNodes(mixed $selector, \DOMNode $context = NULL): array {
+    public function getTargetNodes(mixed $selector, ?\DOMNode $context = NULL): array {
       if ($nodes = $this->getNodeList($selector)) {
         return $nodes;
       }
diff --git a/src/FluentDOM/Nodes/Fetcher.php b/src/FluentDOM/Nodes/Fetcher.php
index 2ce0e079..1cc291f7 100644
--- a/src/FluentDOM/Nodes/Fetcher.php
+++ b/src/FluentDOM/Nodes/Fetcher.php
@@ -40,7 +40,7 @@ public function __construct(Nodes $nodes) {
      * @throws \InvalidArgumentException
      */
     public function fetch(
-      string $expression, callable $filter = NULL, callable $stopAt = NULL, int $options = 0
+      string $expression, ?callable $filter = NULL, ?callable $stopAt = NULL, int $options = 0
     ): array {
       if ($this->validateContextIgnore($expression, $options)) {
         $nodes = $this->fetchFor(
@@ -105,9 +105,9 @@ private function unique(array $nodes, int $options): array {
      */
     private function fetchFor(
       string $expression,
-      \DOMNode $context = NULL,
-      callable $filter = NULL,
-      callable $stopAt = NULL,
+      ?\DOMNode $context = NULL,
+      ?callable $filter = NULL,
+      ?callable $stopAt = NULL,
       int $options = 0
     ): array {
       $nodes = $this->fetchNodes($expression, $context, $options);
@@ -124,7 +124,7 @@ private function fetchFor(
      * @throws \InvalidArgumentException
      */
     private function fetchNodes(
-      string $expression, \DOMNode $context = NULL, int $options = 0
+      string $expression, ?\DOMNode $context = NULL, int $options = 0
     ): array {
       $nodes = $this->_nodes->xpath($expression, $context);
       if (!$nodes instanceof \DOMNodeList) {
@@ -148,7 +148,7 @@ private function fetchNodes(
      * @return array
      */
     private function filterNodes(
-      array $nodes, callable $filter = NULL, callable $stopAt = NULL, int $options = 0
+      array $nodes, ?callable $filter = NULL, ?callable $stopAt = NULL, int $options = 0
     ): array {
       $result = [];
       foreach ($nodes as $index => $node) {
@@ -179,7 +179,7 @@ private function filterNodes(
      * @return bool[]
      */
     private function getNodeStatus(
-      \DOMNode $node, int $index, callable $filter = NULL, callable $stopAt = NULL
+      \DOMNode $node, int $index, ?callable $filter = NULL, ?callable $stopAt = NULL
     ): array {
       return [
         empty($filter) || $filter($node, $index),
diff --git a/src/FluentDOM/Query.php b/src/FluentDOM/Query.php
index d6063e49..deaa6611 100644
--- a/src/FluentDOM/Query.php
+++ b/src/FluentDOM/Query.php
@@ -265,7 +265,7 @@ private function applyToSelector(
      * @example ../examples/Query/add.php Usage Examples: FluentDOM::add()
      */
     public function add(
-      string|iterable $selector, iterable|\DOMNode $context = NULL
+      string|iterable $selector, iterable|\DOMNode|null $context = NULL
     ): static {
       $result = $this->spawn($this);
       if (NULL !== $context) {
@@ -302,7 +302,7 @@ public function addBack(): static {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/children.php Usage Examples: FluentDOM\Query::children()
      */
-    public function children(string $selector = NULL): static {
+    public function children(?string $selector = NULL): static {
       return $this->fetch('*', $selector, NULL, Nodes\Fetcher::UNIQUE);
     }
 
@@ -315,7 +315,7 @@ public function children(string $selector = NULL): static {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/closest.php Usage Example: FluentDOM\Query::closest()
      */
-    public function closest(string|callable $selector, iterable $context = NULL): static {
+    public function closest(string|callable $selector, ?iterable $context = NULL): static {
       $context = $context ? $this->spawn($context) : $this;
       return $context->fetch(
         'ancestor-or-self::*',
@@ -397,7 +397,7 @@ public function first(): static {
      *
      * @param int|NULL $position optional offset of a single element to get.
      */
-    public function get(int $position = NULL): array|\DOMNode|NULL {
+    public function get(?int $position = NULL): array|\DOMNode|NULL {
       if (NULL === $position) {
         return $this->_nodes;
       }
@@ -514,7 +514,7 @@ function (\DOMNode $node, $index) use ($callback) {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/next.php Usage Example: FluentDOM\Query::next()
      */
-    public function next(string $selector = NULL): static {
+    public function next(?string $selector = NULL): static {
       return $this->fetch(
         'following-sibling::node()[
           self::* or (self::text() and normalize-space(.) != "")
@@ -533,7 +533,7 @@ public function next(string $selector = NULL): static {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/nextAll.php Usage Example: FluentDOM\Query::nextAll()
      */
-    public function nextAll(string $selector = NULL): static {
+    public function nextAll(?string $selector = NULL): static {
       return $this->fetch(
         'following-sibling::*|following-sibling::text()[normalize-space(.) != ""]',
         $selector
@@ -548,7 +548,7 @@ public function nextAll(string $selector = NULL): static {
      * @throws \LogicException
      * @throws \InvalidArgumentException
      */
-    public function nextUntil(string $selector = NULL, string $filter = NULL): static {
+    public function nextUntil(?string $selector = NULL, ?string $filter = NULL): static {
       return $this->fetch(
         'following-sibling::*|following-sibling::text()[normalize-space(.) != ""]',
         $filter,
@@ -579,7 +579,7 @@ public function parent(): static {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/parents.php Usage Example: FluentDOM\Query::parents()
      */
-    public function parents(string $selector = NULL): static {
+    public function parents(?string $selector = NULL): static {
       return $this->fetch('ancestor::*', $selector, NULL, Fetcher::REVERSE);
     }
 
@@ -591,7 +591,7 @@ public function parents(string $selector = NULL): static {
      * @throws \LogicException
      * @throws \InvalidArgumentException
      */
-    public function parentsUntil(string $stopAt = NULL, string $filter = NULL): static {
+    public function parentsUntil(?string $stopAt = NULL, ?string $filter = NULL): static {
       return $this->fetch('ancestor::*', $filter, $stopAt, Nodes\Fetcher::REVERSE);
     }
 
@@ -604,7 +604,7 @@ public function parentsUntil(string $stopAt = NULL, string $filter = NULL): stat
      * @throws \InvalidArgumentException
      * @example ../examples/Query/prev.php Usage Example: FluentDOM\Query::prev()
      */
-    public function prev(string $selector = NULL): static {
+    public function prev(?string $selector = NULL): static {
       return $this->fetch(
         'preceding-sibling::node()[
           self::* or (self::text() and normalize-space(.) != "")
@@ -623,7 +623,7 @@ public function prev(string $selector = NULL): static {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/prevAll.php Usage Example: FluentDOM\Query::prevAll()
      */
-    public function prevAll(string $selector = NULL): static {
+    public function prevAll(?string $selector = NULL): static {
       return $this->fetch(
         'preceding-sibling::*|preceding-sibling::text()[normalize-space(.) != ""]',
         $selector,
@@ -640,7 +640,7 @@ public function prevAll(string $selector = NULL): static {
      * @throws \LogicException
      * @throws \InvalidArgumentException
      */
-    public function prevUntil(string $selector = NULL, string $filter = NULL): static {
+    public function prevUntil(?string $selector = NULL, ?string $filter = NULL): static {
       return $this->fetch(
         'preceding-sibling::*|preceding-sibling::text()[normalize-space(.) != ""]',
         $filter,
@@ -669,7 +669,7 @@ public function reverse(): static {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/siblings.php Usage Example: FluentDOM\Query::siblings()
      */
-    public function siblings(string $selector = NULL): static {
+    public function siblings(?string $selector = NULL): static {
       return $this->fetch(
         'preceding-sibling::*|
          preceding-sibling::text()[normalize-space(.) != ""]|
@@ -688,7 +688,7 @@ public function siblings(string $selector = NULL): static {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/slice.php Usage Example: FluentDOM\Query::slice()
      */
-    public function slice(int $start, int $end = NULL): static {
+    public function slice(int $start, ?int $end = NULL): static {
       $result = $this->spawn();
       if ($end === NULL) {
         $result->push(\array_slice($this->_nodes, $start));
@@ -922,7 +922,7 @@ function ($targetNode, $contentNodes) {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/remove.php Usage Example: FluentDOM\Query::remove()
      */
-    public function remove(string $selector = NULL): static {
+    public function remove(?string $selector = NULL): static {
       $result = $this->spawn();
       foreach ($this->_nodes as $node) {
         if (
@@ -942,7 +942,7 @@ public function remove(string $selector = NULL): static {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/text.php Usage Example: FluentDOM\Query::text()
      */
-    public function text(string|callable $text = NULL): string|static {
+    public function text(string|callable|null $text = NULL): string|static {
       if (NULL !== $text) {
         $callback = Constraints::filterCallable($text);
         foreach ($this->_nodes as $index => $node) {
@@ -961,7 +961,7 @@ public function text(string|callable $text = NULL): string|static {
      * Replace the parent nodes of the current selection with
      * their child nodes and remove the parent nodes.
      */
-    public function unwrap(string $selector = NULL): static {
+    public function unwrap(?string $selector = NULL): static {
       $parents = $this->parent();
       if (NULL !== $selector) {
         $parents = $parents->filter($selector);
@@ -1086,7 +1086,7 @@ public function wrapInner(string|\DOMNode|iterable|callable $content): static {
      * @throws \LogicException
      * @example ../examples/Query/xml.php Usage Example: FluentDOM::xml()
      */
-    public function xml(string|callable $xml = NULL): string|static {
+    public function xml(string|callable|null $xml = NULL): string|static {
       return $this->content(
         $xml,
         function ($node) {
@@ -1110,7 +1110,7 @@ function ($node, $fragment) {
      * @throws \InvalidArgumentException
      * @throws InvalidFragmentLoader
      */
-    public function outerXml(string|callable $xml = NULL): string|static {
+    public function outerXml(string|callable|null $xml = NULL): string|static {
       return $this->outerContent(
         $xml,
         function ($node) {
@@ -1131,7 +1131,7 @@ function ($xml) {
      * @throws \InvalidArgumentException
      * @throws InvalidFragmentLoader
      */
-    public function outerHtml(string|callable $html = NULL): string|static {
+    public function outerHtml(string|callable|null $html = NULL): string|static {
       return $this->outerContent(
         $html,
         function ($node) {
@@ -1152,7 +1152,7 @@ function ($html) {
      * @throws \InvalidArgumentException
      * @throws InvalidFragmentLoader
      */
-    public function html(string|callable $html = NULL): string|static {
+    public function html(string|callable|null $html = NULL): string|static {
       return $this->content(
         $html,
         function (\DOMNode $parent) {
@@ -1224,7 +1224,7 @@ function (\DOMNode $node, $fragment) {
     /**
      * @throws \InvalidArgumentException
      */
-    private function getNamesList(string|array $names = NULL): ?array {
+    private function getNamesList(string|array|null $names = NULL): ?array {
       if (\is_array($names)) {
         return $names;
       }
@@ -1242,7 +1242,7 @@ private function getNamesList(string|array $names = NULL): ?array {
      */
     private function getSetterValues(
       string|iterable $name,
-      string|float|int|callable $value = NULL
+      string|float|int|callable|null $value = NULL
     ): iterable {
       return is_string($name) ?  [$name => $value] : $name;
     }
@@ -1368,7 +1368,7 @@ public function removeClass(string|callable $class = ''): static {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/toggleClass.php Usage Example: FluentDOM\Query::toggleClass()
      */
-    public function toggleClass(string|callable $class, bool $switch = NULL): static {
+    public function toggleClass(string|callable $class, ?bool $switch = NULL): static {
       $callback = Constraints::filterCallable($class);
       $this->each(
         function (\DOMElement $node, $index) use ($class, $switch, $callback) {
@@ -1405,7 +1405,7 @@ function (\DOMElement $node, $index) use ($class, $switch, $callback) {
      * toggles the specified classes if the switch is NULL.
      */
     private function changeClassString(
-      string $current, string $toggle, bool $switch = NULL
+      string $current, string $toggle, ?bool $switch = NULL
     ): ?string {
       $currentClasses = \array_flip(
         \preg_split('(\s+)', \trim($current), 0, PREG_SPLIT_NO_EMPTY) ?: []
@@ -1441,7 +1441,7 @@ private function changeClassString(
      * @throws \InvalidArgumentException
      */
     public function css(
-      string|array|Query\Css\Properties $property = NULL,
+      string|array|Query\Css\Properties|null $property = NULL,
       callable|float|int|string|NULL ...$arguments
     ): NULL|string|static {
       if (\is_string($property) && \count($arguments) === 0) {
@@ -1508,7 +1508,7 @@ function (\DOMElement $node) use ($values) {
      * @throws \InvalidArgumentException
      * @example ../examples/Query/removeData.php Usage Example: FluentDOM\Query::removeData()
      */
-    public function removeData(string|array $name = NULL): static {
+    public function removeData(string|array|null $name = NULL): static {
       $names = $this->getNamesList($name);
       $this->each(
         function ($node) use ($names) {
@@ -1532,7 +1532,7 @@ function ($node) use ($names) {
      * Validate if the element has an data attributes attached. If it is called without an
      * actual $element parameter, it will check the first matched node.
      */
-    public function hasData(\DOMElement $element = NULL): bool {
+    public function hasData(?\DOMElement $element = NULL): bool {
       if ($element || ($element = $this->getFirstElement())) {
         $data = new Query\Data($element);
         return \count($data) > 0;
diff --git a/src/FluentDOM/Query/Css/Properties.php b/src/FluentDOM/Query/Css/Properties.php
index 5e2b6e9d..6a31bccf 100644
--- a/src/FluentDOM/Query/Css/Properties.php
+++ b/src/FluentDOM/Query/Css/Properties.php
@@ -148,7 +148,7 @@ public function offsetUnset(mixed $offset): void {
      * Compile value argument into a string (it can be an callback)
      */
     public function compileValue(
-      mixed $value, \DOMElement $node, int $index, string $currentValue = NULL
+      mixed $value, \DOMElement $node, int $index, ?string $currentValue = NULL
     ): string {
       if (!\is_string($value) && \is_callable($value, TRUE)) {
         return (string)$value($node, $index, $currentValue);
diff --git a/src/FluentDOM/Utility/Namespaces.php b/src/FluentDOM/Utility/Namespaces.php
index d16ee022..ecc5ff9d 100644
--- a/src/FluentDOM/Utility/Namespaces.php
+++ b/src/FluentDOM/Utility/Namespaces.php
@@ -29,7 +29,7 @@ class Namespaces implements NamespaceResolver, \ArrayAccess, \IteratorAggregate,
 
     private array $_stash = [];
 
-    public function __construct(iterable $namespaces = NULL) {
+    public function __construct(?iterable $namespaces = NULL) {
       if (NULL !== $namespaces) {
         $this->assign($namespaces);
       }
diff --git a/src/FluentDOM/Utility/QualifiedName.php b/src/FluentDOM/Utility/QualifiedName.php
index 397e77c9..63ab895e 100644
--- a/src/FluentDOM/Utility/QualifiedName.php
+++ b/src/FluentDOM/Utility/QualifiedName.php
@@ -69,7 +69,7 @@ private function setQName(string $name): void {
      *
      * @throws \UnexpectedValueException
      */
-    private function isNCName(string $name, int $offset = 0, string $fullName = NULL): void {
+    private function isNCName(string $name, int $offset = 0, ?string $fullName = NULL): void {
       $nameStartChar =
         'A-Z_a-z'.
         '\\x{C0}-\\x{D6}\\x{D8}-\\x{F6}\\x{F8}-\\x{2FF}\\x{370}-\\x{37D}'.
diff --git a/src/FluentDOM/Utility/ResourceWrapper.php b/src/FluentDOM/Utility/ResourceWrapper.php
index c03450d1..a67883ad 100644
--- a/src/FluentDOM/Utility/ResourceWrapper.php
+++ b/src/FluentDOM/Utility/ResourceWrapper.php
@@ -84,7 +84,7 @@ public function url_stat(
 
     public function stream_open(
       /** @noinspection PhpUnusedParameterInspection */
-      string $path, string $mode, int $options, string &$opened_path = NULL
+      string $path, string $mode, int $options, ?string &$opened_path = NULL
     ): bool {
       [$protocol, $id] = \explode('://', $path);
       $context = \stream_context_get_options($this->context);
diff --git a/src/FluentDOM/XMLReader.php b/src/FluentDOM/XMLReader.php
index 7c5ff33a..eb2165ea 100644
--- a/src/FluentDOM/XMLReader.php
+++ b/src/FluentDOM/XMLReader.php
@@ -46,7 +46,7 @@ public function registerNamespace(string $prefix, string $namespaceURI): void {
      * a namespace prefix it will be resolved using the registered namespaces.
      */
     public function next(
-      string $name = NULL, string $namespaceURI = NULL, callable $filter = NULL
+      ?string $name = NULL, ?string $namespaceURI = NULL, ?callable $filter = NULL
     ): bool {
       if (NULL !== $name) {
         [$localName, $namespaceURI, $ignoreNamespace] = $this->prepareCondition($name, $namespaceURI);
@@ -80,7 +80,7 @@ public function next(
      * @param callable|NULL $filter
      * @return bool
      */
-    public function read(string $name = NULL, string $namespaceURI = NULL, callable $filter = NULL): bool {
+    public function read(?string $name = NULL, ?string $namespaceURI = NULL, ?callable $filter = NULL): bool {
       if (NULL !== $name) {
         [$localName, $namespaceURI, $ignoreNamespace] = $this->prepareCondition($name, $namespaceURI);
         while (parent::read()) {
@@ -141,7 +141,7 @@ public function expand($baseNode = NULL): \DOMNode|FALSE {
      * @param string|NULL $namespaceURI
      * @return array
      */
-    private function prepareCondition(string $name, string $namespaceURI = NULL): array {
+    private function prepareCondition(string $name, ?string $namespaceURI = NULL): array {
       if (NULL !== $namespaceURI) {
         $localName = $name;
         $ignoreNamespace = FALSE;
@@ -160,7 +160,7 @@ private function prepareCondition(string $name, string $namespaceURI = NULL): ar
      * @return bool
      * @throws InvalidArgumentType
      */
-    public function attachStream($stream, string $encoding = NULL, int $options = 0): bool {
+    public function attachStream($stream, ?string $encoding = NULL, int $options = 0): bool {
       if (!\is_resource($stream)) {
         throw new InvalidArgument('stream', 'resource');
       }
diff --git a/src/FluentDOM/XMLReader/Iterator.php b/src/FluentDOM/XMLReader/Iterator.php
index e4123bfa..3434d28b 100644
--- a/src/FluentDOM/XMLReader/Iterator.php
+++ b/src/FluentDOM/XMLReader/Iterator.php
@@ -40,7 +40,7 @@ class Iterator implements \Iterator {
      * Iterator constructor.
      */
     public function __construct(
-      XMLReader $reader, string $name = NULL, callable $filter = NULL
+      XMLReader $reader, ?string $name = NULL, ?callable $filter = NULL
     ) {
       $this->_reader = $reader;
       $this->_name = $name;
@@ -77,7 +77,7 @@ public function next(): void {
      * @return bool
      */
     protected function move(
-      XMLReader $reader, string $name = NULL, callable $filter = NULL
+      XMLReader $reader, ?string $name = NULL, ?callable $filter = NULL
     ): bool {
       while ($reader->read($name, NULL, $filter)) {
         if ($reader->nodeType !== \XMLReader::END_ELEMENT) {
diff --git a/src/FluentDOM/XMLReader/SiblingIterator.php b/src/FluentDOM/XMLReader/SiblingIterator.php
index f97eab05..8058377e 100644
--- a/src/FluentDOM/XMLReader/SiblingIterator.php
+++ b/src/FluentDOM/XMLReader/SiblingIterator.php
@@ -24,7 +24,7 @@
   class SiblingIterator extends Iterator {
 
     protected function move(
-      XMLReader $reader, string $name = NULL, callable $filter = NULL
+      XMLReader $reader, ?string $name = NULL, ?callable $filter = NULL
     ): bool {
       return ($this->key() < 0)
         ? $reader->read($name, NULL, $filter)
diff --git a/src/FluentDOM/XMLWriter.php b/src/FluentDOM/XMLWriter.php
index 429d0a56..8a069abc 100644
--- a/src/FluentDOM/XMLWriter.php
+++ b/src/FluentDOM/XMLWriter.php
@@ -54,7 +54,7 @@ public function startElement(string $name): bool {
       return $this->startElementNS((string)$prefix, $localName, $namespaceURI);
     }
 
-    public function writeElement(string $name, string $content = NULL): bool {
+    public function writeElement(string $name, ?string $content = NULL): bool {
       list($prefix, $localName) = QualifiedName::split($name);
       $namespaceURI = $this->_namespaces->resolveNamespace((string)$prefix);
       return $this->writeElementNS((string)$prefix, $localName, $namespaceURI, $content);
@@ -72,7 +72,7 @@ public function startElementNS(?string $prefix, string $name, ?string $namespace
     }
 
     public function writeElementNS(
-      ?string $prefix, string $name, ?string $namespace, string $content = NULL
+      ?string $prefix, string $name, ?string $namespace, ?string $content = NULL
     ): bool {
       if ($this->_xmlnsStack->isDefined($prefix, $namespace)) {
         return parent::writeElement(empty($prefix) ? $name : $prefix.':'.$name, $content);
diff --git a/src/FluentDOM/XMLWriter/NamespaceDefinition.php b/src/FluentDOM/XMLWriter/NamespaceDefinition.php
index b420785c..3f532333 100644
--- a/src/FluentDOM/XMLWriter/NamespaceDefinition.php
+++ b/src/FluentDOM/XMLWriter/NamespaceDefinition.php
@@ -20,7 +20,7 @@ class NamespaceDefinition implements \IteratorAggregate, NamespaceResolver {
     private int $_indent;
     private Namespaces $_namespaces;
 
-    public function __construct(iterable $inherit = NULL) {
+    public function __construct(?iterable $inherit = NULL) {
       $this->_indent = 0;
       $this->_namespaces = new Namespaces($inherit);
     }
diff --git a/tests/FluentDOM/NodesTest.php b/tests/FluentDOM/NodesTest.php
index 07e77a5e..225ef6ef 100644
--- a/tests/FluentDOM/NodesTest.php
+++ b/tests/FluentDOM/NodesTest.php
@@ -20,7 +20,7 @@
   require_once __DIR__.'/TestCase.php';
 
   class Nodes_TestProxy extends Nodes {
-    public function matches(string $selector, \DOMNode $context = NULL): bool {
+    public function matches(string $selector, ?\DOMNode $context = NULL): bool {
       return parent::matches($selector, $context);
     }
   }
@@ -1170,7 +1170,7 @@ public function testGetSerializerFactoriesInitializesFromStaticClass(): void {
      * @param \DOMNode|NULL $selection
      * @return LoaderResult|MockObject
      */
-    public function createLoaderResultMock(Document $document, \DOMNode $selection = NULL): LoaderResult|MockObject {
+    public function createLoaderResultMock(Document $document, ?\DOMNode $selection = NULL): LoaderResult|MockObject {
       $result = $this
         ->getMockBuilder(Loader\LoaderResult::class)
         ->disableOriginalConstructor()