diff --git a/.github/workflows/phpunit.yml b/.github/workflows/phpunit.yml
index 5feb06ea..42a6481c 100644
--- a/.github/workflows/phpunit.yml
+++ b/.github/workflows/phpunit.yml
@@ -16,7 +16,7 @@ jobs:
           - "lowest"
           - "highest"
         php-version:
-          - "7.4"
+          - "8.0"
         operating-system:
           - "ubuntu-latest"
 
diff --git a/composer.json b/composer.json
index 945fe2bf..4cd8ef00 100644
--- a/composer.json
+++ b/composer.json
@@ -22,15 +22,16 @@
     }
   },
   "require": {
-    "php": ">=7.4",
+    "php": ">=8.0",
     "nikic/php-parser": "^4.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^8.0"
+    "phpunit/phpunit": "^9.0",
+    "tracy/tracy": "^2.10"
   },
   "extra": {
     "branch-alias": {
-      "dev-master": "3.x-dev"
+      "dev-master": "4.x-dev"
     }
   }
 }
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 80281fee..88f4b939 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,19 +1,13 @@
 <?xml version="1.0" encoding="UTF-8"?>
-
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="./vendor/phpunit/phpunit/phpunit.xsd"
-         colors="true"
-         bootstrap="./vendor/autoload.php"
-        >
-    <testsuites>
-        <testsuite name="Parser Reflection Test Suite">
-            <directory>./tests/</directory>
-        </testsuite>
-    </testsuites>
-
-    <filter>
-        <whitelist>
-            <directory>./src/</directory>
-        </whitelist>
-    </filter>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd" colors="true" bootstrap="./vendor/autoload.php">
+  <coverage>
+    <include>
+      <directory>./src/</directory>
+    </include>
+  </coverage>
+  <testsuites>
+    <testsuite name="Parser Reflection Test Suite">
+      <directory>./tests/</directory>
+    </testsuite>
+  </testsuites>
 </phpunit>
diff --git a/src/ReflectionClassConstant.php b/src/ReflectionClassConstant.php
index 7649ae3d..bb14c626 100644
--- a/src/ReflectionClassConstant.php
+++ b/src/ReflectionClassConstant.php
@@ -200,7 +200,7 @@ public function __toString()
         ];
         $value = $this->getValue();
         $type  = gettype($value);
-        if (PHP_VERSION_ID >= 70300 && isset($typeMap[$type])) {
+        if (isset($typeMap[$type])) {
             $type = $typeMap[$type];
         }
         $valueType = new ReflectionType($type, null, true);
diff --git a/src/ReflectionFunction.php b/src/ReflectionFunction.php
index 8813591d..7a3d74c3 100644
--- a/src/ReflectionFunction.php
+++ b/src/ReflectionFunction.php
@@ -77,11 +77,11 @@ public function getClosure()
     /**
      * {@inheritDoc}
      */
-    public function invoke($args = null)
+    public function invoke(mixed ...$args)
     {
         $this->initializeInternalReflection();
 
-        return parent::invoke(...func_get_args());
+        return parent::invoke(...$args);
     }
 
     /**
diff --git a/src/ReflectionMethod.php b/src/ReflectionMethod.php
index 71badc85..49276775 100644
--- a/src/ReflectionMethod.php
+++ b/src/ReflectionMethod.php
@@ -112,11 +112,10 @@ public function __toString()
         }
 
         return sprintf(
-            "%sMethod [ <user%s%s%s>%s%s%s %s method %s ] {\n  @@ %s %d - %d{$paramFormat}{$returnFormat}\n}\n",
+            "%sMethod [ <user%s%s>%s%s%s %s method %s ] {\n  @@ %s %d - %d{$paramFormat}{$returnFormat}\n}\n",
             $this->getDocComment() ? $this->getDocComment() . "\n" : '',
             $prototype ? ", overwrites {$prototypeClass}, prototype {$prototypeClass}" : '',
             $this->isConstructor() ? ', ctor' : '',
-            $this->isDestructor() ? ', dtor' : '',
             $this->isFinal() ? ' final' : '',
             $this->isStatic() ? ' static' : '',
             $this->isAbstract() ? ' abstract' : '',
@@ -203,17 +202,17 @@ public function getPrototype()
     /**
      * {@inheritDoc}
      */
-    public function invoke($object, $args = null)
+    public function invoke(?object $object, mixed ...$args): mixed
     {
         $this->initializeInternalReflection();
 
-        return parent::invoke(...func_get_args());
+        return parent::invoke($object, ...$args);
     }
 
     /**
      * {@inheritDoc}
      */
-    public function invokeArgs($object, array $args)
+    public function invokeArgs(?object $object, array $args): mixed
     {
         $this->initializeInternalReflection();
 
diff --git a/src/ReflectionProperty.php b/src/ReflectionProperty.php
index 6daad7f9..05ecc556 100644
--- a/src/ReflectionProperty.php
+++ b/src/ReflectionProperty.php
@@ -113,11 +113,35 @@ public function __debugInfo(): array
      */
     public function __toString()
     {
+        $defaultValueDisplay = '';
+
+        if ($this->isDefault()) {
+            $this->__initialize();
+            $defaultValue = $this->getDefaultValue();
+            if (is_string($defaultValue)) {
+                if (strlen($defaultValue) > 18) {
+                    $defaultValue = substr($defaultValue, 0, 15) . '...';
+                }
+
+                $defaultValue = "'" . $defaultValue . "'";
+            }
+
+            if ($defaultValue === null) {
+                $defaultValue = "NULL";
+            }
+
+            if (is_array($defaultValue)) {
+                $defaultValueDisplay = '= Array';
+            } else {
+                $defaultValueDisplay = '= ' . $defaultValue;
+            }
+        }
+
         return sprintf(
-            "Property [%s %s $%s ]\n",
-            $this->isStatic() ? '' : ($this->isDefault() ? ' <default>' : ' <dynamic>'),
+            "Property [ %s $%s %s ]\n",
             implode(' ', Reflection::getModifierNames($this->getModifiers())),
-            $this->getName()
+            $this->getName(),
+            $defaultValueDisplay
         );
     }
 
diff --git a/src/ReflectionType.php b/src/ReflectionType.php
index 5e13d7a1..fd4572de 100644
--- a/src/ReflectionType.php
+++ b/src/ReflectionType.php
@@ -14,6 +14,7 @@
 
 use ReflectionNamedType;
 use ReflectionType as BaseReflectionType;
+use ReflectionUnionType;
 
 /**
  * ReflectionType implementation
@@ -92,8 +93,8 @@ public static function convertToDisplayType(BaseReflectionType $type)
 
         $displayType = ltrim($displayType, '\\');
 
-        if ($type->allowsNull()) {
-            $displayType .= ' or NULL';
+        if ($type->allowsNull() && ! $type instanceof ReflectionUnionType) {
+            $displayType = '?' . $displayType;
         }
 
         return $displayType;
diff --git a/src/Traits/ReflectionClassLikeTrait.php b/src/Traits/ReflectionClassLikeTrait.php
index 200c88e3..5e1b6dad 100644
--- a/src/Traits/ReflectionClassLikeTrait.php
+++ b/src/Traits/ReflectionClassLikeTrait.php
@@ -221,7 +221,7 @@ public function getConstant($name)
     /**
      * {@inheritDoc}
      */
-    public function getConstants()
+    public function getConstants(?int $filter = null): array
     {
         if (!isset($this->constants)) {
             $this->constants = $this->recursiveCollect(
@@ -417,10 +417,6 @@ public function getModifiers()
             $modifiers += \ReflectionClass::IS_FINAL;
         }
 
-        if (PHP_VERSION_ID < 70000 && $this->isTrait()) {
-            $modifiers += \ReflectionClass::IS_EXPLICIT_ABSTRACT;
-        }
-
         if ($this->classLikeNode instanceof Class_ && $this->classLikeNode->isAbstract()) {
             $modifiers += \ReflectionClass::IS_EXPLICIT_ABSTRACT;
         }
@@ -553,7 +549,7 @@ public function getReflectionConstant($name)
     /**
      * @inheritDoc
      */
-    public function getReflectionConstants()
+    public function getReflectionConstants(?int $filter = null): array
     {
         if (!isset($this->classConstants)) {
             $directClassConstants = ReflectionClassConstant::collectFromClassNode(
@@ -965,12 +961,12 @@ public function newInstanceArgs(array $args = [])
      *
      * @return object
      */
-    public function newInstanceWithoutConstructor($args = null)
+    public function newInstanceWithoutConstructor()
     {
         $function = __FUNCTION__;
         $this->initializeInternalReflection();
 
-        return parent::$function($args);
+        return parent::$function();
     }
 
     /**
diff --git a/tests/AbstractTestCase.php b/tests/AbstractTestCase.php
index 36335a45..c05cc145 100644
--- a/tests/AbstractTestCase.php
+++ b/tests/AbstractTestCase.php
@@ -71,16 +71,9 @@ public function testCoverAllMethods()
     public function getFilesToAnalyze()
     {
         $files = ['PHP5.5' => [__DIR__ . '/Stub/FileWithClasses55.php']];
-
-        if (PHP_VERSION_ID >= 50600) {
-            $files['PHP5.6'] = [__DIR__ . '/Stub/FileWithClasses56.php'];
-        }
-        if (PHP_VERSION_ID >= 70000) {
-            $files['PHP7.0'] = [__DIR__ . '/Stub/FileWithClasses70.php'];
-        }
-        if (PHP_VERSION_ID >= 70100) {
-            $files['PHP7.1'] = [__DIR__ . '/Stub/FileWithClasses71.php'];
-        }
+        $files['PHP5.6'] = [__DIR__ . '/Stub/FileWithClasses56.php'];
+        $files['PHP7.0'] = [__DIR__ . '/Stub/FileWithClasses70.php'];
+        $files['PHP7.1'] = [__DIR__ . '/Stub/FileWithClasses71.php'];
 
         return $files;
     }
diff --git a/tests/ReflectionClassTest.php b/tests/ReflectionClassTest.php
index 0b0e4c74..8e51057e 100644
--- a/tests/ReflectionClassTest.php
+++ b/tests/ReflectionClassTest.php
@@ -30,7 +30,7 @@ class ReflectionClassTest extends AbstractTestCase
      */
     public function testGetModifiers($fileName)
     {
-        if (PHP_VERSION_ID >= 70400) {
+        if (PHP_VERSION_ID >= 80000) {
             $this->markTestSkipped('TODO: Fix mapping and logic of modifiers');
         }
         $mask =
diff --git a/tests/ReflectionFunctionTest.php b/tests/ReflectionFunctionTest.php
index a34ec96a..1239983f 100644
--- a/tests/ReflectionFunctionTest.php
+++ b/tests/ReflectionFunctionTest.php
@@ -34,9 +34,7 @@ public function testGeneralInfoGetters()
             'returnsReference', 'getClosureScopeClass', 'getClosureThis'
         ];
 
-        if (PHP_VERSION_ID >= 70000) {
-            $allNameGetters[] = 'hasReturnType';
-        }
+        $allNameGetters[] = 'hasReturnType';
 
         foreach ($this->parsedRefFile->getFileNamespaces() as $fileNamespace) {
             foreach ($fileNamespace->getFunctions() as $refFunction) {
@@ -72,7 +70,7 @@ public function testCoverAllMethods()
         }
 
         if ($allMissedMethods) {
-            $this->markTestIncomplete('Methods ' . join($allMissedMethods, ', ') . ' are not implemented');
+            $this->markTestIncomplete('Methods ' . implode(', ', $allMissedMethods) . ' are not implemented');
         }
     }
 
@@ -105,10 +103,6 @@ public function testInvokeArgsMethod()
 
     public function testGetReturnTypeMethod()
     {
-        if (PHP_VERSION_ID < 70000) {
-            $this->markTestSkipped('Test available only for PHP7.0 and newer');
-        }
-
         $fileName = stream_resolve_include_path(__DIR__ . self::STUB_FILE70);
 
         $reflectionFile = new ReflectionFile($fileName);
@@ -129,12 +123,7 @@ public function testGetReturnTypeMethod()
                     $originalReturnType = $originalRefFunction->getReturnType();
                     $this->assertSame($originalReturnType->allowsNull(), $parsedReturnType->allowsNull());
                     $this->assertSame($originalReturnType->isBuiltin(), $parsedReturnType->isBuiltin());
-                    // TODO: To prevent deprecation error in tests
-                    if (PHP_VERSION_ID < 70400) {
-                        $this->assertSame($originalReturnType->__toString(), $parsedReturnType->__toString());
-                    } else {
-                        $this->assertSame($originalReturnType->getName(), $parsedReturnType->__toString());
-                    }
+                    $this->assertSame($originalReturnType->getName(), $parsedReturnType->__toString());
                 } else {
                     $this->assertSame(
                         $originalRefFunction->getReturnType(),
diff --git a/tests/ReflectionMethodTest.php b/tests/ReflectionMethodTest.php
index 08fe2456..9d84b183 100644
--- a/tests/ReflectionMethodTest.php
+++ b/tests/ReflectionMethodTest.php
@@ -146,14 +146,9 @@ protected function getGettersToCheck()
             'getNumberOfRequiredParameters', 'returnsReference', 'getClosureScopeClass', 'getClosureThis'
         ];
 
-        if (PHP_VERSION_ID >= 50600) {
-            $allNameGetters[] = 'isVariadic';
-            $allNameGetters[] = 'isGenerator';
-        }
-
-        if (PHP_VERSION_ID >= 70000) {
-            $allNameGetters[] = 'hasReturnType';
-        }
+        $allNameGetters[] = 'isVariadic';
+        $allNameGetters[] = 'isGenerator';
+        $allNameGetters[] = 'hasReturnType';
 
         return $allNameGetters;
     }
diff --git a/tests/ReflectionParameterTest.php b/tests/ReflectionParameterTest.php
index 7f142bdc..5210fb7a 100644
--- a/tests/ReflectionParameterTest.php
+++ b/tests/ReflectionParameterTest.php
@@ -34,12 +34,8 @@ public function testGeneralInfoGetters($fileName)
         $onlyWithDefaultValues = array_flip([
             'getDefaultValue', 'getDefaultValueConstantName', 'isDefaultValueConstant'
         ]);
-        if (PHP_VERSION_ID >= 50600) {
-            $allNameGetters[] = 'isVariadic';
-        }
-        if (PHP_VERSION_ID >= 70000) {
-            $allNameGetters[] = 'hasType';
-        }
+        $allNameGetters[] = 'isVariadic';
+        $allNameGetters[] = 'hasType';
 
         foreach ($this->parsedRefFile->getFileNamespaces() as $fileNamespace) {
             foreach ($fileNamespace->getFunctions() as $refFunction) {
@@ -75,13 +71,8 @@ public function testGeneralInfoGetters($fileName)
     public function fileProvider()
     {
         $files = ['PHP5.5' => [__DIR__ . '/Stub/FileWithParameters55.php']];
-
-        if (PHP_VERSION_ID >= 50600) {
-            $files['PHP5.6'] = [__DIR__ . '/Stub/FileWithParameters56.php'];
-        }
-        if (PHP_VERSION_ID >= 70000) {
-            $files['PHP7.0'] = [__DIR__ . '/Stub/FileWithParameters70.php'];
-        }
+        $files['PHP5.6'] = [__DIR__ . '/Stub/FileWithParameters56.php'];
+        $files['PHP7.0'] = [__DIR__ . '/Stub/FileWithParameters70.php'];
 
         return $files;
     }
@@ -253,15 +244,12 @@ public function testCoverAllMethods()
         }
 
         if ($allMissedMethods) {
-            $this->markTestIncomplete('Methods ' . join($allMissedMethods, ', ') . ' are not implemented');
+            $this->markTestIncomplete('Methods ' . implode(', ', $allMissedMethods) . ' are not implemented');
         }
     }
 
     public function testGetTypeMethod()
     {
-        if (PHP_VERSION_ID < 70000) {
-            $this->markTestSkipped('Test available only for PHP7.0 and newer');
-        }
         $this->setUpFile(__DIR__ . '/Stub/FileWithParameters70.php');
 
         foreach ($this->parsedRefFile->getFileNamespaces() as $fileNamespace) {
@@ -282,12 +270,7 @@ public function testGetTypeMethod()
                         $originalReturnType = $originalRefParameter->getType();
                         $this->assertSame($originalReturnType->allowsNull(), $parsedReturnType->allowsNull(), $message);
                         $this->assertSame($originalReturnType->isBuiltin(), $parsedReturnType->isBuiltin(), $message);
-                        // TODO: To prevent deprecation error in tests
-                        if (PHP_VERSION_ID < 70400) {
-                            $this->assertSame($originalReturnType->__toString(), $parsedReturnType->__toString(), $message);
-                        } else {
-                            $this->assertSame($originalReturnType->getName(), $parsedReturnType->__toString(), $message);
-                        }
+                        $this->assertSame($originalReturnType->getName(), $parsedReturnType->__toString(), $message);
                     } else {
                         $this->assertSame(
                             $originalRefParameter->getType(),
diff --git a/tests/ReflectionTypeTest.php b/tests/ReflectionTypeTest.php
index 79910567..e22e5ea4 100644
--- a/tests/ReflectionTypeTest.php
+++ b/tests/ReflectionTypeTest.php
@@ -9,12 +9,9 @@ class ReflectionTypeTest extends TestCase
 {
     protected function setUp(): void
     {
-        if (PHP_VERSION_ID >= 70000) {
-            include_once (__DIR__ . '/Stub/FileWithClasses70.php');
-        }
-        if (PHP_VERSION_ID >= 70100) {
-            include_once (__DIR__ . '/Stub/FileWithClasses71.php');
-        }
+        include_once (__DIR__ . '/Stub/FileWithClasses70.php');
+        include_once (__DIR__ . '/Stub/FileWithClasses71.php');
+        include_once (__DIR__ . '/Stub/FileWithClasses80.php');
     }
 
     /**
@@ -32,12 +29,7 @@ public function testTypeConvertToDisplayTypeWithNativeType()
         $this->assertCount(2, $nativeParamRefArr);
         $this->assertEquals(\ReflectionParameter::class, get_class($nativeParamRefArr[0]));
         $nativeTypeRef = $nativeParamRefArr[0]->getType();
-        // TODO: To prevent deprecation error in tests
-        if (PHP_VERSION_ID < 70400) {
-            $this->assertEquals('string', (string)$nativeTypeRef);
-        } else {
-            $this->assertEquals('string', $nativeTypeRef->getName());
-        }
+        $this->assertEquals('string', $nativeTypeRef->getName());
         $this->assertStringNotContainsString('\\', get_class($nativeTypeRef));
         $this->assertInstanceOf(\ReflectionType::class, $nativeTypeRef);
         $this->assertEquals('string', \Go\ParserReflection\ReflectionType::convertToDisplayType($nativeTypeRef));
@@ -58,15 +50,10 @@ public function testTypeConvertToDisplayTypeWithNullableNativeType()
         $this->assertCount(2, $nativeParamRefArr);
         $this->assertEquals(\ReflectionParameter::class, get_class($nativeParamRefArr[0]));
         $nativeTypeRef = $nativeParamRefArr[0]->getType();
-        // TODO: To prevent deprecation error in tests
-        if (PHP_VERSION_ID < 70400) {
-            $this->assertEquals('string', (string)$nativeTypeRef);
-        } else {
-            $this->assertEquals('string', $nativeTypeRef->getName());
-        }
+        $this->assertEquals('string', $nativeTypeRef->getName());
         $this->assertStringNotContainsString('\\', get_class($nativeTypeRef));
         $this->assertInstanceOf(\ReflectionType::class, $nativeTypeRef);
-        $this->assertEquals('string or NULL', \Go\ParserReflection\ReflectionType::convertToDisplayType($nativeTypeRef));
+        $this->assertEquals('?string', \Go\ParserReflection\ReflectionType::convertToDisplayType($nativeTypeRef));
     }
 
     /**
@@ -85,14 +72,31 @@ public function testTypeConvertToDisplayTypeImplicitlyNullable()
         $this->assertEquals(\ReflectionParameter::class, get_class($nativeParamRefArr[0]));
         $nativeTypeRef = $nativeParamRefArr[0]->getType();
         $this->assertTrue($nativeTypeRef->allowsNull());
-        // TODO: To prevent deprecation error in tests
-        if (PHP_VERSION_ID < 70400) {
-            $this->assertEquals('string', (string)$nativeTypeRef);
-        } else {
-            $this->assertEquals('string', $nativeTypeRef->getName());
-        }
+        $this->assertEquals('string', $nativeTypeRef->getName());
         $this->assertStringNotContainsString('\\', get_class($nativeTypeRef));
         $this->assertInstanceOf(\ReflectionType::class, $nativeTypeRef);
-        $this->assertEquals('string or NULL', \Go\ParserReflection\ReflectionType::convertToDisplayType($nativeTypeRef));
+        $this->assertEquals('?string', \Go\ParserReflection\ReflectionType::convertToDisplayType($nativeTypeRef));
+    }
+
+    /**
+     * Testing convertToDisplayType() with native \ReflectionType
+     *
+     * We're already testing it with Go\ParserReflection\ReflectionType
+     * elsewhere.
+     */
+    public function testTypeConvertToDisplayTypeImplicitlyUnionNullable()
+    {
+        $nativeClassRef = new \ReflectionClass('Go\\ParserReflection\\Stub\\ClassWithPhp80Features');
+        $nativeMethodRef = $nativeClassRef->getMethod('acceptsStringArrayDefaultToNull');
+        $this->assertEquals(\ReflectionMethod::class, get_class($nativeMethodRef));
+        $nativeParamRefArr = $nativeMethodRef->getParameters();
+        $this->assertCount(1, $nativeParamRefArr);
+        $this->assertEquals(\ReflectionParameter::class, get_class($nativeParamRefArr[0]));
+        $nativeTypeRef = $nativeParamRefArr[0]->getType();
+        $this->assertTrue($nativeTypeRef->allowsNull());
+        $this->assertEquals('array|string|null', $nativeTypeRef);
+        $this->assertStringNotContainsString('\\', get_class($nativeTypeRef));
+        $this->assertInstanceOf(\ReflectionType::class, $nativeTypeRef);
+        $this->assertEquals('array|string|null', \Go\ParserReflection\ReflectionType::convertToDisplayType($nativeTypeRef));
     }
 }
diff --git a/tests/Stub/FileWithClasses80.php b/tests/Stub/FileWithClasses80.php
new file mode 100644
index 00000000..9784aa27
--- /dev/null
+++ b/tests/Stub/FileWithClasses80.php
@@ -0,0 +1,19 @@
+<?php
+/**
+ * Parser Reflection API
+ *
+ * @copyright Copyright 2016, Lisachenko Alexander <lisachenko.it@gmail.com>
+ *
+ * This source file is subject to the license that is bundled
+ * with this source code in the file LICENSE.
+ */
+declare(strict_types=1);
+
+namespace Go\ParserReflection\Stub;
+
+use Go\ParserReflection\{ReflectionMethod, ReflectionProperty as P};
+
+class ClassWithPhp80Features
+{
+    public function acceptsStringArrayDefaultToNull(array|string $iterable = null) : array {}
+}