diff --git a/.github/workflows/qa.yaml b/.github/workflows/qa.yaml
new file mode 100644
index 00000000..55d48e32
--- /dev/null
+++ b/.github/workflows/qa.yaml
@@ -0,0 +1,119 @@
+name: Quality and Assurance
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: true
+      matrix:
+        php-versions: ['7.1', '8.1']
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Setup PHP, extensions and composer with shivammathur/setup-php
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-versions }}
+          extensions: mbstring, xml, ctype, iconv, intl, pdo, pdo_mysql, dom, filter, gd, iconv, json, mbstring, pdo
+        env:
+          update: true
+
+      - name: Validate composer.json
+        run: composer validate --strict
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: Install Composer dependencies
+        run: composer install --no-progress --prefer-dist --optimize-autoloader
+
+      - name: Install Lowest Composer dependencies
+        if: "matrix.php-versions < '8.1'"
+        run: composer update --prefer-lowest --no-progress --prefer-dist --optimize-autoloader
+
+      - name: Install PHPUnit
+        run: vendor/bin/simple-phpunit install
+
+      - name: Cache composer dependencies
+        uses: actions/cache@v1
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
+          restore-keys: ${{ runner.os }}-composer-
+          php-version: ${{ matrix.php-versions }}
+
+  qa:
+    name: Quality
+    needs: build
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: true
+      matrix:
+        php-versions: ['8.1']
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Setup PHP, extensions and composer with shivammathur/setup-php
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-versions }}
+          extensions: mbstring, xml, ctype, intl, dom, filter
+          tools: symfony
+        env:
+          update: true
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: Install Composer dependencies
+        run: composer install --no-progress --prefer-dist --optimize-autoloader
+
+      - name: PHP-CS-Fixer
+        run: symfony php ./vendor/bin/php-cs-fixer fix --dry-run --using-cache=no --verbose --diff
+
+  test:
+    name: Tests
+    needs: build
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: true
+      matrix:
+        php-versions: ['7.1', '8.1']
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Setup PHP, extensions and composer with shivammathur/setup-php
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-versions }}
+          extensions: mbstring, xml, ctype, intl, dom, filter
+          tools: symfony
+        env:
+          update: true
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: Install Composer dependencies
+        run: composer install --no-progress --prefer-dist --optimize-autoloader
+
+      - name: Install Lowest Composer dependencies
+        if: "matrix.php-versions < '8.1'"
+        run: composer install --no-progress --prefer-dist --optimize-autoloader
+
+      - name: Execute unit tests
+        run: symfony php ./vendor/bin/simple-phpunit
diff --git a/.gitignore b/.gitignore
index 3714b9b6..55240f9e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,4 @@
-.php_cs.cache
+.php-cs-fixer.cache
 .phpunit.result.cache
 phpunit.xml
 composer.lock
diff --git a/.php_cs.dist b/.php-cs-fixer.dist.php
similarity index 65%
rename from .php_cs.dist
rename to .php-cs-fixer.dist.php
index ac731140..7de6fa49 100644
--- a/.php_cs.dist
+++ b/.php-cs-fixer.dist.php
@@ -1,11 +1,11 @@
 <?php
   
-$finder = PhpCsFixer\Finder::create()
+$finder = (new PhpCsFixer\Finder())
     ->exclude('Resources')
     ->in(__DIR__)
 ;
 
-return PhpCsFixer\Config::create()
+return (new PhpCsFixer\Config())
     ->setRules([
         '@Symfony' => true,
     ])
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index e0dc3bd7..00000000
--- a/.travis.yml
+++ /dev/null
@@ -1,68 +0,0 @@
-language: php
-
-sudo: false
-
-dist: xenial
-
-env:
-    global:
-        - COMPOSER_MEMORY_LIMIT=-1
-        - SYMFONY_PHPUNIT_DIR=$HOME/.phpunit-bridge
-
-cache:
-    directories:
-        - $HOME/.composer/cache
-        - $HOME/.phpunit-bridge
-
-jobs:
-    include:
-        # Lowest
-        - php: 7.1
-          env: COMPOSER_FLAGS="--prefer-lowest" SYMFONY_DEPRECATIONS_HELPER=weak
-        - php: 7.1
-          env: SYMFONY_REQUIRE="4.3.*" COMPOSER_FLAGS="--prefer-lowest" SYMFONY_DEPRECATIONS_HELPER=weak
-
-        # Stable
-        - php: 7.2
-          env: SYMFONY_REQUIRE="3.4.*" COMPOSER_FLAGS="--prefer-stable"
-        - php: 7.3
-          env: SYMFONY_REQUIRE="4.4.*" COMPOSER_FLAGS="--prefer-stable"
-        - php: 7.4
-          env: SYMFONY_REQUIRE="5.0.*" COMPOSER_FLAGS="--prefer-stable"
-        - php: 8.0
-          env: SYMFONY_REQUIRE="5.0.*" COMPOSER_FLAGS="--prefer-stable" SYMFONY_DEPRECATIONS_HELPER=weak
-
-        # Dev
-        - php: 7.4
-          env: STABILITY=dev
-
-        # QA
-        - stage: QA
-          name: PHP CS Fixer
-          php: 7.4
-          script: vendor/bin/php-cs-fixer fix --dry-run --diff
-        - name: Coverage
-          php: 7.4
-          before_script:
-            - mv ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini{.disabled,}
-            - echo "xdebug.mode = coverage" >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini
-            - if [[ ! $(php -m | grep -si xdebug) ]]; then echo "xdebug required for coverage"; exit 1; fi
-          script:
-            - ./vendor/bin/simple-phpunit -v --coverage-text
-
-    allow_failures:
-        - env: STABILITY=dev
-
-before_install:
-  - mv ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini{,.disabled} || echo "xdebug not available"
-  - composer self-update
-  - composer global require --no-progress --no-scripts --no-plugins symfony/flex
-
-install:
-  - composer update --prefer-dist --no-interaction $COMPOSER_FLAGS
-  - vendor/bin/simple-phpunit install
-
-script:
-  - if [[ -v $STABILITY ]]; then composer config minimum-stability $STABILITY; fi;
-  - composer validate --strict --no-check-lock
-  - vendor/bin/simple-phpunit -v
diff --git a/CHANGELOG b/CHANGELOG
index 62e68f86..308e9413 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,9 @@
+## Version 4.0 (04/2022)
+
+ * [BC Break] Drop support for Symfony < 4.4
+ * add support for Symfony 6.x
+ * add support for PHPUnit 10.x
+
 ## Version 3.0 (12/2019)
 
  * [BC break] Dropped support for PHP 5.x. PHP 7.1 minimum required.
diff --git a/README.md b/README.md
index 25af43e4..ae39ef4f 100644
--- a/README.md
+++ b/README.md
@@ -11,44 +11,13 @@ This bundle integrates [HTMLPurifier][] into Symfony.
 
 ## Installation
 
-## Symfony 3.4 and above (using Composer)
-
-Require the bundle in your composer.json file:
-
-```json
-{
-    "require": {
-        "exercise/htmlpurifier-bundle": "*"
-    }
-}
-```
-
 Install the bundle:
 
 ```bash
 $ composer require exercise/htmlpurifier-bundle
 ```
 
-Register the bundle in Symfony 3:
-
-```php
-// app/AppKernel.php
-
-public function registerBundles()
-{
-    return [
-        // ...
-        new Exercise\HTMLPurifierBundle\ExerciseHTMLPurifierBundle(),
-    ];
-}
-```
-
-## Configuration in Symfony 3
-
-The configuration is the same as the following section, but the path should be
-`app/config.yml` instead.
-
-## Configuration in Symfony 4 and up
+## Configuration
 
 If you do not explicitly configure this bundle, an HTMLPurifier service will be
 defined as `exercise_html_purifier.default`. This behavior is the same as if you
@@ -117,7 +86,7 @@ services:
     exercise_html_purifier.default: '@App\Html\CustomHtmlPurifier'
 ```
 
-### Argument binding (Symfony >= 4.4)
+### Argument binding
 
 The bundle also leverages the alias argument binding for each profile. So the
 following config:
@@ -353,5 +322,5 @@ previous, and "all" could define its own rules too.
 
 ## Contributing
 
-PRs are welcomed :). Please target the `2.0` branch for bug fixes and `master`
+PRs are welcomed :). Please target the `3.x` branch for bug fixes and `master`
 for new features.
diff --git a/composer.json b/composer.json
index dddff0ff..e2c50218 100644
--- a/composer.json
+++ b/composer.json
@@ -13,15 +13,15 @@
     ],
     "require": {
         "php": "^7.1.3 || ^8.0.0",
-        "ezyang/htmlpurifier": "~4.0",
-        "symfony/config": "~3.4 || ~4.0 || ^5.0",
-        "symfony/dependency-injection": "~3.4.1 || ^4.0.1 || ^5.0",
-        "symfony/http-kernel": "~3.4.1 || ^4.0.1 || ^5.0"
+        "ezyang/htmlpurifier": "~4.14",
+        "symfony/config": "~4.4 || ^5.0 || ^6.0",
+        "symfony/dependency-injection": "^4.4 || ^5.0 || ^6.0",
+        "symfony/http-kernel": "^4.4 || ^5.0 || ^6.0"
     },
     "require-dev": {
-        "friendsofphp/php-cs-fixer": "^2.0",
-        "symfony/form": "~3.4.1 || ^4.0.1 || ^5.0",
-        "symfony/phpunit-bridge": "4.4.*",
+        "friendsofphp/php-cs-fixer": "^3.0",
+        "symfony/form": "^4.4 || ^5.0 || ^6.0",
+        "symfony/phpunit-bridge": "^6.0",
         "twig/twig": "^1.35.0 || ^2.4.4 || ^3.0"
     },
     "autoload": {
@@ -35,7 +35,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "3.0.x-dev"
+            "dev-master": "4.x-dev"
         }
     }
 }
diff --git a/src/CacheWarmer/SerializerCacheWarmer.php b/src/CacheWarmer/SerializerCacheWarmer.php
index a0f9c762..bfa42f13 100644
--- a/src/CacheWarmer/SerializerCacheWarmer.php
+++ b/src/CacheWarmer/SerializerCacheWarmer.php
@@ -26,9 +26,8 @@ class SerializerCacheWarmer implements CacheWarmerInterface
     private $filesystem;
 
     /**
-     * @param string[]                       $paths
-     * @param string[]                       $profiles
-     * @param HTMLPurifiersRegistryInterface $registry Used to build cache within bundle runtime
+     * @param string[] $paths
+     * @param string[] $profiles
      */
     public function __construct(array $paths, array $profiles, HTMLPurifiersRegistryInterface $registry, Filesystem $filesystem)
     {
@@ -41,7 +40,7 @@ public function __construct(array $paths, array $profiles, HTMLPurifiersRegistry
     /**
      * {@inheritdoc}
      */
-    public function warmUp($cacheDir)
+    public function warmUp($cacheDir): array
     {
         foreach ($this->paths as $path) {
             $this->filesystem->remove($path); // clean previous cache
@@ -52,12 +51,14 @@ public function warmUp($cacheDir)
             // Will build the configuration
             $this->registry->get($profile)->purify("<div style=\"background:url('http://www.example.com/x.gif');\">");
         }
+
+        return [];
     }
 
     /**
      * {@inheritdoc}
      */
-    public function isOptional()
+    public function isOptional(): bool
     {
         return false;
     }
diff --git a/src/DependencyInjection/Compiler/HTMLPurifierPass.php b/src/DependencyInjection/Compiler/HTMLPurifierPass.php
index ce744e84..16ed073a 100644
--- a/src/DependencyInjection/Compiler/HTMLPurifierPass.php
+++ b/src/DependencyInjection/Compiler/HTMLPurifierPass.php
@@ -12,12 +12,12 @@
 
 class HTMLPurifierPass implements CompilerPassInterface
 {
-    const PURIFIER_TAG = 'exercise.html_purifier';
+    public const PURIFIER_TAG = 'exercise.html_purifier';
 
     /**
      * {@inheritdoc}
      */
-    public function process(ContainerBuilder $container)
+    public function process(ContainerBuilder $container): void
     {
         if (!$container->hasAlias(HTMLPurifiersRegistryInterface::class)) {
             return;
diff --git a/src/DependencyInjection/Configuration.php b/src/DependencyInjection/Configuration.php
index 8ecee7ad..4b534279 100644
--- a/src/DependencyInjection/Configuration.php
+++ b/src/DependencyInjection/Configuration.php
@@ -11,16 +11,10 @@ class Configuration implements ConfigurationInterface
     /**
      * {@inheritdoc}
      */
-    public function getConfigTreeBuilder()
+    public function getConfigTreeBuilder(): TreeBuilder
     {
         $treeBuilder = new TreeBuilder('exercise_html_purifier');
-
-        if (method_exists($treeBuilder, 'getRootNode')) {
-            $rootNode = $treeBuilder->getRootNode();
-        } else {
-            // BC layer for symfony/config 4.1 and older
-            $rootNode = $treeBuilder->root('exercise_html_purifier');
-        }
+        $rootNode = $treeBuilder->getRootNode();
 
         $rootNode
             ->children()
diff --git a/src/DependencyInjection/ExerciseHTMLPurifierExtension.php b/src/DependencyInjection/ExerciseHTMLPurifierExtension.php
index 76965aaf..74b067e4 100644
--- a/src/DependencyInjection/ExerciseHTMLPurifierExtension.php
+++ b/src/DependencyInjection/ExerciseHTMLPurifierExtension.php
@@ -14,7 +14,10 @@
 
 class ExerciseHTMLPurifierExtension extends Extension
 {
-    public function load(array $configs, ContainerBuilder $container)
+    /**
+     * {@inheritdoc}
+     */
+    public function load(array $configs, ContainerBuilder $container): void
     {
         $loader = new XmlFileLoader($container, new FileLocator(__DIR__.'/../Resources/config'));
 
@@ -87,7 +90,7 @@ public function load(array $configs, ContainerBuilder $container)
         ;
     }
 
-    public function getAlias()
+    public function getAlias(): string
     {
         return 'exercise_html_purifier';
     }
diff --git a/src/ExerciseHTMLPurifierBundle.php b/src/ExerciseHTMLPurifierBundle.php
index 31966ee9..f22d1b0b 100644
--- a/src/ExerciseHTMLPurifierBundle.php
+++ b/src/ExerciseHTMLPurifierBundle.php
@@ -11,7 +11,7 @@ class ExerciseHTMLPurifierBundle extends Bundle
     /**
      * {@inheritdoc}
      */
-    public function build(ContainerBuilder $container)
+    public function build(ContainerBuilder $container): void
     {
         $container->addCompilerPass(new HTMLPurifierPass());
     }
diff --git a/src/Form/Listener/HTMLPurifierListener.php b/src/Form/Listener/HTMLPurifierListener.php
index 98d86dd1..dfbf7420 100644
--- a/src/Form/Listener/HTMLPurifierListener.php
+++ b/src/Form/Listener/HTMLPurifierListener.php
@@ -25,7 +25,7 @@ public function purifySubmittedData(FormEvent $event): void
             return; // because we don't want to handle it here
         }
 
-        if (0 === strlen($submittedData = trim($data))) {
+        if (0 === strlen($submittedData = trim((string) $data))) {
             if ($submittedData !== $data) {
                 $event->setData($submittedData);
             }
@@ -39,7 +39,7 @@ public function purifySubmittedData(FormEvent $event): void
     /**
      * {@inheritdoc}
      */
-    public static function getSubscribedEvents()
+    public static function getSubscribedEvents(): array
     {
         return [
             FormEvents::PRE_SUBMIT => ['purifySubmittedData', /* as soon as possible */ 1000000],
diff --git a/src/Form/TypeExtension/HTMLPurifierTextTypeExtension.php b/src/Form/TypeExtension/HTMLPurifierTextTypeExtension.php
index 0b88e03e..90815dd7 100644
--- a/src/Form/TypeExtension/HTMLPurifierTextTypeExtension.php
+++ b/src/Form/TypeExtension/HTMLPurifierTextTypeExtension.php
@@ -20,14 +20,6 @@ public function __construct(HTMLPurifiersRegistryInterface $registry)
         $this->purifiersRegistry = $registry;
     }
 
-    /**
-     * {@inheritdoc}
-     */
-    public function getExtendedType()
-    {
-        return TextType::class;
-    }
-
     /**
      * {@inheritdoc}
      */
@@ -39,7 +31,7 @@ public static function getExtendedTypes(): iterable
     /**
      * {@inheritdoc}
      */
-    public function configureOptions(OptionsResolver $resolver)
+    public function configureOptions(OptionsResolver $resolver): void
     {
         $resolver
             ->setDefaults([
@@ -69,7 +61,7 @@ public function configureOptions(OptionsResolver $resolver)
     /**
      * {@inheritdoc}
      */
-    public function buildForm(FormBuilderInterface $builder, array $options)
+    public function buildForm(FormBuilderInterface $builder, array $options): void
     {
         if ($options['purify_html']) {
             $builder->addEventSubscriber(
diff --git a/src/HTMLPurifierConfigFactory.php b/src/HTMLPurifierConfigFactory.php
index ce0657b2..9ba5d1aa 100644
--- a/src/HTMLPurifierConfigFactory.php
+++ b/src/HTMLPurifierConfigFactory.php
@@ -62,7 +62,7 @@ public static function create(
      * This build should never happen on runtime, since purifiers cache should
      * be generated during warm up.
      */
-    public static function buildHTMLDefinition(\HTMLPurifier_Definition $def, array $attributes, array $elements, array $blankElements): void
+    public static function buildHTMLDefinition(\HTMLPurifier_HTMLDefinition $def, array $attributes, array $elements, array $blankElements): void
     {
         foreach ($attributes as $elementName => $rule) {
             foreach ($rule as $attributeName => $definition) {
diff --git a/src/Twig/HTMLPurifierExtension.php b/src/Twig/HTMLPurifierExtension.php
index 713596d8..aceafb77 100644
--- a/src/Twig/HTMLPurifierExtension.php
+++ b/src/Twig/HTMLPurifierExtension.php
@@ -10,7 +10,7 @@ class HTMLPurifierExtension extends AbstractExtension
     /**
      * {@inheritdoc}
      */
-    public function getFilters()
+    public function getFilters(): array
     {
         return [
             new TwigFilter('purify', [HTMLPurifierRuntime::class, 'purify'], ['is_safe' => ['html']]),
diff --git a/tests/CacheWarmer/SerializerCacheWarmerTest.php b/tests/CacheWarmer/SerializerCacheWarmerTest.php
index 42856753..b21ec241 100644
--- a/tests/CacheWarmer/SerializerCacheWarmerTest.php
+++ b/tests/CacheWarmer/SerializerCacheWarmerTest.php
@@ -9,14 +9,14 @@
 
 class SerializerCacheWarmerTest extends TestCase
 {
-    public function testShouldBeRequired()
+    public function testShouldBeRequired(): void
     {
         $cacheWarmer = new SerializerCacheWarmer([], [], $this->createMock(HTMLPurifiersRegistryInterface::class), new Filesystem());
 
         $this->assertFalse($cacheWarmer->isOptional());
     }
 
-    public function testWarmUpShouldCreatePaths()
+    public function testWarmUpShouldCreatePaths(): void
     {
         $fs = new Filesystem();
         $path = sys_get_temp_dir().DIRECTORY_SEPARATOR.'html_purifier';
@@ -28,14 +28,14 @@ public function testWarmUpShouldCreatePaths()
         $this->assertFalse($fs->exists($path));
 
         $cacheWarmer = new SerializerCacheWarmer([$path], [], $this->createMock(HTMLPurifiersRegistryInterface::class), $fs);
-        $cacheWarmer->warmUp(null);
+        $cacheWarmer->warmUp('');
 
         $this->assertTrue($fs->exists($path));
 
         $fs->remove($path);
     }
 
-    public function testWarmUpShouldCallPurifyForEachProfile()
+    public function testWarmUpShouldCallPurifyForEachProfile(): void
     {
         $purifier = $this->createMock(\HTMLPurifier::class);
         $purifier->expects($this->exactly(2))
@@ -45,18 +45,11 @@ public function testWarmUpShouldCallPurifyForEachProfile()
         $registry = $this->createMock(HTMLPurifiersRegistryInterface::class);
         $registry->expects($this->exactly(2))
             ->method('get')
+            ->withConsecutive(['first'], ['second'])
             ->willReturn($purifier)
         ;
-        $registry->expects($this->at(0))
-            ->method('get')
-            ->with('first')
-        ;
-        $registry->expects($this->at(1))
-            ->method('get')
-            ->with('second')
-        ;
 
         $cacheWarmer = new SerializerCacheWarmer([], ['first', 'second'], $registry, new Filesystem());
-        $cacheWarmer->warmUp(null);
+        $cacheWarmer->warmUp('');
     }
 }
diff --git a/tests/DependencyInjection/Compiler/HTMLPurifierPassTest.php b/tests/DependencyInjection/Compiler/HTMLPurifierPassTest.php
index 7c484f2f..f5ef7090 100644
--- a/tests/DependencyInjection/Compiler/HTMLPurifierPassTest.php
+++ b/tests/DependencyInjection/Compiler/HTMLPurifierPassTest.php
@@ -16,7 +16,7 @@
 
 class HTMLPurifierPassTest extends TestCase
 {
-    /** @var ContainerBuilder|MockObject */
+    /** @var ContainerBuilder|MockObject|null */
     private $container;
 
     protected function setUp(): void
@@ -34,7 +34,7 @@ protected function tearDown(): void
         $this->container = null;
     }
 
-    public function testProcessOnlyIfRegistryInterfaceIsDefined()
+    public function testProcessOnlyIfRegistryInterfaceIsDefined(): void
     {
         $this->container->expects($this->once())
             ->method('hasAlias')
@@ -50,7 +50,7 @@ public function testProcessOnlyIfRegistryInterfaceIsDefined()
         $pass->process($this->container);
     }
 
-    public function testProcess()
+    public function testProcess(): void
     {
         $container = new ContainerBuilder();
         $purifier = $container->register(DummyPurifier::class)
@@ -71,7 +71,7 @@ public function testProcess()
         $this->assertSame(DummyPurifier::class, (string) $map['test']->getValues()[0]);
     }
 
-    public function testProcessDoNothingIfRegistryIsNotDefined()
+    public function testProcessDoNothingIfRegistryIsNotDefined(): void
     {
         $this->container
             ->expects($this->once())
@@ -94,7 +94,7 @@ public function testProcessDoNothingIfRegistryIsNotDefined()
         $pass->process($this->container);
     }
 
-    public function testProcessFailsIfTaggedServiceMissesProfileName()
+    public function testProcessFailsIfTaggedServiceMissesProfileName(): void
     {
         $container = new ContainerBuilder();
         $container->register(DummyPurifier::class)
diff --git a/tests/DependencyInjection/ExerciseHTMLPurifierExtensionTest.php b/tests/DependencyInjection/ExerciseHTMLPurifierExtensionTest.php
index 274e5431..ffaa3e2e 100644
--- a/tests/DependencyInjection/ExerciseHTMLPurifierExtensionTest.php
+++ b/tests/DependencyInjection/ExerciseHTMLPurifierExtensionTest.php
@@ -17,17 +17,17 @@ class ExerciseHTMLPurifierExtensionTest extends TestCase
     private const DEFAULT_CACHE_PATH = '%kernel.cache_dir%/htmlpurifier';
 
     /**
-     * @var ContainerBuilder
+     * @var ContainerBuilder|null
      */
     private $container;
 
     /**
-     * @var ExerciseHTMLPurifierExtension
+     * @var ExerciseHTMLPurifierExtension|null
      */
     private $extension;
 
     /**
-     * @var array
+     * @var array|null
      */
     private $defaultConfig;
 
@@ -48,7 +48,7 @@ public function tearDown(): void
         $this->container = null;
     }
 
-    public function testShouldLoadDefaultConfiguration()
+    public function testShouldLoadDefaultConfiguration(): void
     {
         $this->extension->load([], $this->container);
 
@@ -57,7 +57,7 @@ public function testShouldLoadDefaultConfiguration()
         $this->assertRegistryHasProfiles(['default']);
     }
 
-    public function testInvalidParent()
+    public function testInvalidParent(): void
     {
         $config = [
             'html_profiles' => [
@@ -80,12 +80,12 @@ public function testInvalidParent()
     /**
      * @dataProvider provideInvalidElementDefinitions
      */
-    public function testInvalidElements(array $elementDefinition)
+    public function testInvalidElements(array $elementDefinition): void
     {
         $config = [
             'html_profiles' => [
                 'default' => [
-                    'elements' => ['a' => []],
+                    'elements' => ['a' => $elementDefinition],
                 ],
             ],
         ];
@@ -104,7 +104,7 @@ public function provideInvalidElementDefinitions(): iterable
         yield 'too many arguments' => [['', '', '', [], [], 'extra argument']];
     }
 
-    public function testShouldAllowOverridingDefaultConfigurationCacheSerializerPath()
+    public function testShouldAllowOverridingDefaultConfigurationCacheSerializerPath(): void
     {
         $config = [
             'default_cache_serializer_path' => null,
@@ -126,7 +126,7 @@ public function testShouldAllowOverridingDefaultConfigurationCacheSerializerPath
         $this->assertRegistryHasProfiles(['default']);
     }
 
-    public function testShouldNotDeepMergeOptions()
+    public function testShouldNotDeepMergeOptions(): void
     {
         $configs = [
             ['html_profiles' => [
@@ -154,7 +154,7 @@ public function testShouldNotDeepMergeOptions()
         $this->assertRegistryHasProfiles(['default']);
     }
 
-    public function testShouldLoadCustomConfiguration()
+    public function testShouldLoadCustomConfiguration(): void
     {
         $config = [
             'html_profiles' => [
@@ -195,7 +195,7 @@ public function testShouldLoadCustomConfiguration()
         $this->assertRegistryHasProfiles($profiles);
     }
 
-    public function testShouldLoadComplexCustomConfiguration()
+    public function testShouldLoadComplexCustomConfiguration(): void
     {
         $defaultConfig = [
             'AutoFormat.AutoParagraph' => true,
@@ -288,7 +288,7 @@ public function testShouldLoadComplexCustomConfiguration()
         $this->assertRegistryHasProfiles($profiles);
     }
 
-    public function testShouldRegisterAliases()
+    public function testShouldRegisterAliases(): void
     {
         if (!method_exists($this->container, 'registerAliasForArgument')) {
             $this->markTestSkipped('Alias arguments binding is not available.');
@@ -373,10 +373,8 @@ public function testShouldRegisterAliases()
     /**
      * Asserts that the named config definition extends the default profile and
      * loads the given options.
-     *
-     * @param string $name
      */
-    private function assertConfigDefinition($name, array $config, array $parents = [], array $attributes = [], array $elements = [], array $blankElements = [])
+    private function assertConfigDefinition(string $name, array $config, array $parents = [], array $attributes = [], array $elements = [], array $blankElements = []): void
     {
         $this->assertTrue($this->container->hasDefinition('exercise_html_purifier.config.'.$name));
 
diff --git a/tests/Form/Listener/HTMLPurifierListenerTest.php b/tests/Form/Listener/HTMLPurifierListenerTest.php
index 14447a60..04dc8240 100644
--- a/tests/Form/Listener/HTMLPurifierListenerTest.php
+++ b/tests/Form/Listener/HTMLPurifierListenerTest.php
@@ -10,7 +10,7 @@
 
 class HTMLPurifierListenerTest extends TestCase
 {
-    public function testPurify()
+    public function testPurify(): void
     {
         $input = 'text';
         $purifiedInput = '<p>text</p>';
@@ -41,7 +41,7 @@ public function testPurify()
         $this->assertSame($purifiedInput, $event->getData());
     }
 
-    public function testPurifyTrimEmptyValues()
+    public function testPurifyTrimEmptyValues(): void
     {
         $input = ' ';
         $trimmedInput = '';
@@ -69,8 +69,10 @@ public function testPurifyTrimEmptyValues()
 
     /**
      * @dataProvider provideInvalidInput
+     *
+     * @param mixed $input
      */
-    public function testPurifyDoNothingForEmptyOrNonScalarData($input)
+    public function testPurifyDoNothingForEmptyOrNonScalarData($input): void
     {
         $registry = $this->createMock(HTMLPurifiersRegistryInterface::class);
         $registry
@@ -101,6 +103,9 @@ public function provideInvalidInput(): iterable
         yield [new \stdClass()];
     }
 
+    /**
+     * @param mixed $data
+     */
     private function getFormEvent($data): FormEvent
     {
         return new FormEvent($this->createMock(FormInterface::class), $data);
diff --git a/tests/Form/TypeExtension/HTMLPurifierTextTypeExtensionTest.php b/tests/Form/TypeExtension/HTMLPurifierTextTypeExtensionTest.php
index 1bddef62..84682181 100644
--- a/tests/Form/TypeExtension/HTMLPurifierTextTypeExtensionTest.php
+++ b/tests/Form/TypeExtension/HTMLPurifierTextTypeExtensionTest.php
@@ -5,6 +5,7 @@
 use Exercise\HTMLPurifierBundle\Form\Listener\HTMLPurifierListener;
 use Exercise\HTMLPurifierBundle\Form\TypeExtension\HTMLPurifierTextTypeExtension;
 use Exercise\HTMLPurifierBundle\HTMLPurifiersRegistryInterface;
+use PHPUnit\Framework\MockObject\MockObject;
 use Symfony\Component\Form\Extension\Core\Type\TextType;
 use Symfony\Component\Form\FormEvents;
 use Symfony\Component\Form\FormInterface;
@@ -13,6 +14,7 @@
 
 class HTMLPurifierTextTypeExtensionTest extends FormIntegrationTestCase
 {
+    /** @var HTMLPurifiersRegistryInterface|MockObject|null */
     private $registry;
 
     protected function setUp(): void
@@ -29,14 +31,14 @@ protected function tearDown(): void
         $this->registry = null;
     }
 
-    protected function getTypeExtensions()
+    protected function getTypeExtensions(): array
     {
         return [
             new HTMLPurifierTextTypeExtension($this->registry),
         ];
     }
 
-    public function testDefaultOptions()
+    public function testDefaultOptions(): void
     {
         $this->registry
             ->expects($this->never())
@@ -56,7 +58,7 @@ public function testDefaultOptions()
         $this->assertFalse($this->hasPurifierListener($form));
     }
 
-    public function testPurifyOptionsNeedDefaultProfile()
+    public function testPurifyOptionsNeedDefaultProfile(): void
     {
         $this->registry
             ->expects($this->once())
@@ -75,7 +77,7 @@ public function testPurifyOptionsNeedDefaultProfile()
         $this->factory->create(TextType::class, null, ['purify_html' => true]);
     }
 
-    public function testDefaultOptionsWhenPurifyIsTrue()
+    public function testDefaultOptionsWhenPurifyIsTrue(): void
     {
         $this->registry
             ->expects($this->once())
@@ -93,7 +95,7 @@ public function testDefaultOptionsWhenPurifyIsTrue()
         $this->assertTrue($this->hasPurifierListener($form));
     }
 
-    public function testInvalidProfile()
+    public function testInvalidProfile(): void
     {
         $this->registry
             ->expects($this->once())
diff --git a/tests/HTMLPurifierConfigFactoryTest.php b/tests/HTMLPurifierConfigFactoryTest.php
index e0a129d6..70dfec9c 100644
--- a/tests/HTMLPurifierConfigFactoryTest.php
+++ b/tests/HTMLPurifierConfigFactoryTest.php
@@ -8,6 +8,7 @@
 
 class HTMLPurifierConfigFactoryTest extends TestCase
 {
+    /** @var string */
     private static $cacheDir;
 
     public static function setUpBeforeClass(): void
@@ -21,14 +22,14 @@ public static function tearDownAfterClass(): void
         (new Filesystem())->remove(self::$cacheDir);
     }
 
-    public function testCreateUseDoesNotBuildDefinitionByDefault()
+    public function testCreateUseDoesNotBuildDefinitionByDefault(): void
     {
         TestHTMLPurifierConfigFactory::create('default', []);
 
         $this->assertSame(0, TestHTMLPurifierConfigFactory::$calledBuild);
     }
 
-    public function testCreateUseSerializedCache()
+    public function testCreateUseSerializedCache(): void
     {
         $configArgs = [
             'test', /* profile */
@@ -53,10 +54,11 @@ public function testCreateUseSerializedCache()
 
 class TestHTMLPurifierConfigFactory extends HTMLPurifierConfigFactory
 {
+    /** @var int */
     public static $calledBuild = 0;
 
     public static function buildHTMLDefinition(
-        \HTMLPurifier_Definition $def,
+        \HTMLPurifier_HTMLDefinition $def,
         array $attributes,
         array $elements,
         array $blankElements
diff --git a/tests/HTMLPurifiersRegistryTest.php b/tests/HTMLPurifiersRegistryTest.php
index d58d6df0..5dcfacea 100644
--- a/tests/HTMLPurifiersRegistryTest.php
+++ b/tests/HTMLPurifiersRegistryTest.php
@@ -3,12 +3,15 @@
 namespace Exercise\HTMLPurifierBundle\Tests;
 
 use Exercise\HTMLPurifierBundle\HTMLPurifiersRegistry;
+use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
 use Psr\Container\ContainerInterface;
 
 class HTMLPurifiersRegistryTest extends TestCase
 {
+    /** @var MockObject|ContainerInterface|null */
     private $locator;
+    /** @var HTMLPurifiersRegistry|null */
     private $registry;
 
     protected function setUp(): void
@@ -32,7 +35,7 @@ public function provideProfiles(): iterable
     /**
      * @dataProvider provideProfiles
      */
-    public function testHas($profile)
+    public function testHas(string $profile): void
     {
         $this->locator->expects($this->once())
             ->method('has')
@@ -46,7 +49,7 @@ public function testHas($profile)
     /**
      * @dataProvider provideProfiles
      */
-    public function testHasNot($profile)
+    public function testHasNot(string $profile): void
     {
         $this->locator->expects($this->once())
             ->method('has')
@@ -60,7 +63,7 @@ public function testHasNot($profile)
     /**
      * @dataProvider provideProfiles
      */
-    public function testGet($profile)
+    public function testGet(string $profile): void
     {
         $purifier = $this->createMock(\HTMLPurifier::class);
 
diff --git a/tests/Twig/HTMLPurifierRuntimeTest.php b/tests/Twig/HTMLPurifierRuntimeTest.php
index e6b03349..21594ddb 100644
--- a/tests/Twig/HTMLPurifierRuntimeTest.php
+++ b/tests/Twig/HTMLPurifierRuntimeTest.php
@@ -11,7 +11,7 @@ class HTMLPurifierRuntimeTest extends TestCase
     /**
      * @dataProvider providePurifierProfiles
      */
-    public function testPurifyFilter($profile)
+    public function testPurifyFilter(string $profile): void
     {
         $input = 'text';
         $purifiedInput = '<p>text</p>';