diff --git a/.gitignore b/.gitignore
index a064c15..1c81cdc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,4 +2,5 @@ build
 composer.lock
 vendor
 phpcs.xml
-phpunit.xml
\ No newline at end of file
+phpunit.xml
+.idea
diff --git a/composer.json b/composer.json
index 6b46a9f..c79d646 100644
--- a/composer.json
+++ b/composer.json
@@ -20,8 +20,9 @@
         }
     ],
     "require": {
-        "php" : "~7.0",
-        "ext-dom": "*"
+        "php" : "~8.1",
+        "ext-dom": "*",
+        "ext-libxml": "*"
     },
     "require-dev": {
         "phpunit/phpunit" : ">=5.4.3",
diff --git a/src/MicrodataDOMDocument.php b/src/MicrodataDOMDocument.php
index bf3c47d..c590cc3 100644
--- a/src/MicrodataDOMDocument.php
+++ b/src/MicrodataDOMDocument.php
@@ -2,10 +2,12 @@
 
 namespace YusufKandemir\MicrodataParser;
 
-class MicrodataDOMDocument extends \DOMDocument
+use DOMDocument;
+use DOMXPath;
+
+class MicrodataDOMDocument extends DOMDocument
 {
-    /** @var \DOMXPath */
-    public $xpath;
+    public DOMXPath $xpath;
 
     /**
      * Get top-level items of the document
@@ -23,11 +25,11 @@ public function getItems() : \DOMNodeList
      * {@inheritdoc}
      * Also assigns $xpath with DOMXPath of freshly loaded DOMDocument
      */
-    public function loadHTML($source, $options = 0)
+    public function loadHTML($source, $options = 0): DOMDocument|bool
     {
         $return = parent::loadHTML($source, $options);
 
-        $this->xpath = new \DOMXPath($this);
+        $this->xpath = new DOMXPath($this);
 
         return $return;
     }
@@ -36,11 +38,11 @@ public function loadHTML($source, $options = 0)
      * {@inheritdoc}
      * Also assigns $xpath with DOMXPath of freshly loaded DOMDocument
      */
-    public function loadHTMLFile($filename, $options = 0)
+    public function loadHTMLFile($filename, $options = 0): DOMDocument|bool
     {
         $return = parent::loadHTMLFile($filename, $options);
 
-        $this->xpath = new \DOMXPath($this);
+        $this->xpath = new DOMXPath($this);
 
         return $return;
     }
diff --git a/src/MicrodataDOMElement.php b/src/MicrodataDOMElement.php
index d5acdb0..2dcc63b 100644
--- a/src/MicrodataDOMElement.php
+++ b/src/MicrodataDOMElement.php
@@ -2,10 +2,13 @@
 
 namespace YusufKandemir\MicrodataParser;
 
+use function array_key_exists;
+use function in_array;
+
 class MicrodataDOMElement extends \DOMElement
 {
     /** @var array "tag name" to "attribute name" mapping */
-    private static $tagNameLookup = [
+    private static array $tagNameLookup = [
         'audio' => 'src',
         'embed' => 'src',
         'iframe' => 'src',
@@ -23,7 +26,7 @@ class MicrodataDOMElement extends \DOMElement
     ];
 
     /** @var array Attributes that have absolute values */
-    private static $absoluteAttributes = ['src', 'href', 'data',];
+    private static array $absoluteAttributes = ['src', 'href', 'data',];
 
     /**
      * @see https://www.w3.org/TR/2018/WD-microdata-20180426/#dfn-item-properties for details of algorithm
@@ -94,11 +97,11 @@ public function getPropertyNames() : array
     /**
      * @see https://www.w3.org/TR/2018/WD-microdata-20180426/#dfn-property-value for details of algorithm
      *
-     * @param callable $absoluteUriHandler
+     * @param callable|null $absoluteUriHandler
      *
      * @return $this|string
      */
-    public function getPropertyValue(callable $absoluteUriHandler = null)
+    public function getPropertyValue(callable $absoluteUriHandler = null): string|static
     {
         if ($this->hasAttribute('itemscope')) {
             return $this;
@@ -110,11 +113,11 @@ public function getPropertyValue(callable $absoluteUriHandler = null)
 
         $value = '';
 
-        if (\array_key_exists($this->tagName, self::$tagNameLookup)) {
+        if (array_key_exists($this->tagName, self::$tagNameLookup)) {
             $attribute = self::$tagNameLookup[$this->tagName];
             $value = $this->getAttribute($attribute);
 
-            if (!empty($value) && \in_array($attribute, self::$absoluteAttributes) && !$this->isAbsoluteUri($value)) {
+            if (!empty($value) && in_array($attribute, self::$absoluteAttributes) && !$this->isAbsoluteUri($value)) {
                 $value = $absoluteUriHandler($value, $this->ownerDocument->documentURI);
             }
         }
@@ -132,7 +135,7 @@ public function getPropertyValue(callable $absoluteUriHandler = null)
      *
      * @return false|int
      */
-    protected function isAbsoluteUri(string $uri)
+    protected function isAbsoluteUri(string $uri): bool|int
     {
         return preg_match("/^\w+:/", trim($uri));
     }
@@ -142,7 +145,7 @@ protected function isAbsoluteUri(string $uri)
      *
      * @return array Result array which contains child ElementNodes
      */
-    protected function getChildElementNodes()
+    protected function getChildElementNodes(): array
     {
         $childNodes = [];
 
@@ -160,9 +163,9 @@ protected function getChildElementNodes()
      *
      * @param string $attributeName Name of the attribute
      *
-     * @return array|array[]|false|string[]
+     * @return array|false
      */
-    public function tokenizeAttribute(string $attributeName)
+    public function tokenizeAttribute(string $attributeName): array|bool
     {
         $attribute = [];
 
@@ -176,15 +179,15 @@ public function tokenizeAttribute(string $attributeName)
     /**
      * Splits given attribute value in space characters to array
      *
+     * @param string $attribute
+     *
+     * @return array|false
      * @see \preg_split() for possible return values and behaviour
      *
      * @see https://www.w3.org/TR/2018/WD-microdata-20180426/#dfn-split-a-string-on-spaces for definition of tokens
      *
-     * @param string $attribute
-     *
-     * @return array[]|false|string[]
      */
-    protected function tokenize(string $attribute)
+    protected function tokenize(string $attribute): array|bool
     {
         return preg_split('/\s+/', trim($attribute));
     }
diff --git a/src/MicrodataParser.php b/src/MicrodataParser.php
index 262ec98..92c59f1 100644
--- a/src/MicrodataParser.php
+++ b/src/MicrodataParser.php
@@ -2,10 +2,12 @@
 
 namespace YusufKandemir\MicrodataParser;
 
+use DOMElement;
+use stdClass;
+
 class MicrodataParser
 {
-    /** @var MicrodataDOMDocument */
-    protected $dom;
+    protected MicrodataDOMDocument $dom;
 
     /**
      * Handler will be called with $value(non-absolute uri string) and $base(base uri) parameters
@@ -26,7 +28,7 @@ class MicrodataParser
      */
     public function __construct(MicrodataDOMDocument $dom, callable $absoluteUriHandler = null)
     {
-        $dom->registerNodeClass(\DOMElement::class, MicrodataDOMElement::class);
+        $dom->registerNodeClass(DOMElement::class, MicrodataDOMElement::class);
 
         $this->dom = $dom;
         $this->absoluteUriHandler = $absoluteUriHandler ?: function ($value, $base) {
@@ -48,9 +50,9 @@ public function toArray() : array
     /**
      * Extracts and converts microdata to object
      *
-     * @return \stdClass
+     * @return stdClass
      */
-    public function toObject() : \stdClass
+    public function toObject() : stdClass
     {
         return $this->extractMicrodata();
     }
@@ -58,24 +60,24 @@ public function toObject() : \stdClass
     /**
      * Extracts and converts microdata to json using \json_encode()
      *
-     * @see \json_encode() to description of parameters and return values
-     *
      * @param int $options
      * @param int $depth
      *
      * @return false|string
+     * @see \json_encode() to description of parameters and return values
+     *
      */
-    public function toJSON($options = 0, $depth = 512)
+    public function toJSON(int $options = 0, int $depth = 512): bool|string
     {
         return json_encode($this->extractMicrodata(), $options, $depth);
     }
 
     /**
-     * @return \stdClass
+     * @return stdClass
      */
-    protected function extractMicrodata() : \stdClass
+    protected function extractMicrodata() : stdClass
     {
-        $result = new \stdClass;
+        $result = new stdClass;
 
         $result->items = [];
 
@@ -92,11 +94,11 @@ protected function extractMicrodata() : \stdClass
      * @param MicrodataDOMElement $item
      * @param array $memory
      *
-     * @return \stdClass
+     * @return stdClass
      */
-    protected function getObject(MicrodataDOMElement $item, $memory = []) : \stdClass
+    protected function getObject(MicrodataDOMElement $item, array $memory = []) : stdClass
     {
-        $result = new \stdClass;
+        $result = new stdClass;
 
         $memory[] = $item;
 
@@ -108,7 +110,7 @@ protected function getObject(MicrodataDOMElement $item, $memory = []) : \stdClas
         }
         // @todo Check if item ids are valid absolute urls or like isbn:xxx
 
-        $properties = new \stdClass;
+        $properties = new stdClass;
 
         foreach ($item->getProperties() as $element) {
             $value = $element->getPropertyValue($this->absoluteUriHandler);
diff --git a/tests/MicrodataParserTest.php b/tests/MicrodataParserTest.php
index d9c4480..ffd2ac9 100644
--- a/tests/MicrodataParserTest.php
+++ b/tests/MicrodataParserTest.php
@@ -2,17 +2,18 @@
 
 namespace YusufKandemir\MicrodataParser\Tests;
 
+use PHPUnit\Framework\TestCase;
 use YusufKandemir\MicrodataParser\MicrodataDOMDocument;
 use YusufKandemir\MicrodataParser\MicrodataParser;
 
-class MicrodataParserTest extends \PHPUnit\Framework\TestCase
+class MicrodataParserTest extends TestCase
 {
-    protected function setUp()
+    protected function setUp() :void
     {
         libxml_use_internal_errors(true); // Ignore warnings of DOMDocument::loadHTML check
     }
 
-    protected function getParser($data)
+    protected function getParser($data): MicrodataParser
     {
         $dom = new MicrodataDOMDocument;
         $dom->loadHTML($data['source']);
@@ -66,7 +67,7 @@ public function testItUsesAbsoluteUriHandlerWhenHandlingAbsoluteUris()
         $resultBefore = $parser->toObject();
         $resultBeforeUri = $resultBefore->items[0]->properties->work[0];
 
-        $this->assertNotContains($baseUri, $resultBeforeUri);
+        $this->assertStringNotContainsString($baseUri, $resultBeforeUri);
 
         $parser->setAbsoluteUriHandler(
             function (string $value, string $base) use ($baseUri) : string {
@@ -77,13 +78,19 @@ function (string $value, string $base) use ($baseUri) : string {
         $resultAfter = $parser->toObject();
         $resultAfterUri = $resultAfter->items[0]->properties->work[0];
 
-        $this->assertContains($baseUri, $resultAfterUri);
+        $this->assertStringContainsString($baseUri, $resultAfterUri);
     }
 
     /**
      * @todo Provide more test data
+     * @return array{
+     *  'W3C Example': array{uri: string, source: string, result: string}[],
+     *  'Itemref & src based tags': array{uri: string, source: string, result: string}[],
+     *  'Object & Data tags': array{uri: string, source: string, result: string}[],
+     *  'Itemid & Content attributes': array{uri: string, source: string, result: string}[]
+     * }
      */
-    public function data()
+    public function data(): array
     {
         return [
             // https://www.w3.org/TR/microdata/#ex-jsonconv
@@ -102,13 +109,20 @@ public function data()
         ];
     }
 
-    private function getTestData($folderName, $sourceName, $resultName)
+    /**
+     * @return array{uri: string, source: string, result: string}
+     */
+    private function getTestData($folderName, $sourceName, $resultName): array
     {
         $folderPath = __DIR__.'/data/'.$folderName.'/';
 
         $source = file_get_contents($folderPath . $sourceName);
         $result = file_get_contents($folderPath . $resultName);
 
+        if ($source === false || $result === false) {
+            throw new \Exception('Could not load test data');
+        }
+
         $uri = '';
         // Set $uri if URI specified in test data
         if (preg_match('/<!-- URI: (.*) -->/', $source, $matches)) {
diff --git a/tests/MicrodataTest.php b/tests/MicrodataTest.php
index d3a9aec..2a87983 100644
--- a/tests/MicrodataTest.php
+++ b/tests/MicrodataTest.php
@@ -2,12 +2,13 @@
 
 namespace YusufKandemir\MicrodataParser\Tests;
 
+use DOMDocument;
 use YusufKandemir\MicrodataParser\Microdata;
 use YusufKandemir\MicrodataParser\MicrodataParser;
 
 class MicrodataTest extends \PHPUnit\Framework\TestCase
 {
-    protected $htmlFileName = __DIR__ . '/data/W3C/source.html';
+    protected string $htmlFileName = __DIR__ . '/data/W3C/source.html';
 
     public function testItCreatesMicrodataParserFromHtml()
     {
@@ -26,7 +27,7 @@ public function testItCreatesMicrodataParserFromHtmlFile()
 
     public function testItCreatesMicrodataParserFromDomDocument()
     {
-        $dom = new \DOMDocument;
+        $dom = new DOMDocument;
         $dom->loadHTMLFile($this->htmlFileName);
 
         $microdata = Microdata::fromDOMDocument($dom);