diff --git a/composer.json b/composer.json
index 79f5fd57541..08ea64e07dd 100644
--- a/composer.json
+++ b/composer.json
@@ -72,6 +72,7 @@
         "symfony/form": "^3.4 || ^4.4 || ^5.1",
         "symfony/framework-bundle": "^4.4 || ^5.1",
         "symfony/http-client": "^4.4 || ^5.1",
+        "symfony/intl": "^3.4 || ^4.4 || ^5.1",
         "symfony/mercure-bundle": "*",
         "symfony/messenger": "^4.4 || ^5.1",
         "symfony/phpunit-bridge": "^5.1.7",
diff --git a/src/Bridge/RamseyUuid/Serializer/UuidDenormalizer.php b/src/Bridge/RamseyUuid/Serializer/UuidDenormalizer.php
index 8a8ff7abe63..a3ea6a812d5 100644
--- a/src/Bridge/RamseyUuid/Serializer/UuidDenormalizer.php
+++ b/src/Bridge/RamseyUuid/Serializer/UuidDenormalizer.php
@@ -21,6 +21,11 @@
 
 final class UuidDenormalizer implements DenormalizerInterface
 {
+    /**
+     * @param mixed|null $format
+     *
+     * @return UuidInterface
+     */
     public function denormalize($data, $type, $format = null, array $context = [])
     {
         try {
@@ -30,7 +35,7 @@ public function denormalize($data, $type, $format = null, array $context = [])
         }
     }
 
-    public function supportsDenormalization($data, $type, $format = null)
+    public function supportsDenormalization($data, $type, $format = null): bool
     {
         return \is_string($data) && is_a($type, UuidInterface::class, true);
     }
diff --git a/src/Bridge/Symfony/Bundle/Command/GraphQlExportCommand.php b/src/Bridge/Symfony/Bundle/Command/GraphQlExportCommand.php
index 2b2402fc54a..4311cd9f1ec 100644
--- a/src/Bridge/Symfony/Bundle/Command/GraphQlExportCommand.php
+++ b/src/Bridge/Symfony/Bundle/Command/GraphQlExportCommand.php
@@ -55,7 +55,7 @@ protected function configure(): void
     /**
      * {@inheritdoc}
      */
-    protected function execute(InputInterface $input, OutputInterface $output)
+    protected function execute(InputInterface $input, OutputInterface $output): int
     {
         $io = new SymfonyStyle($input, $output);
 
diff --git a/src/Bridge/Symfony/Bundle/Command/OpenApiCommand.php b/src/Bridge/Symfony/Bundle/Command/OpenApiCommand.php
index f117899f705..9a7c8e46c1f 100644
--- a/src/Bridge/Symfony/Bundle/Command/OpenApiCommand.php
+++ b/src/Bridge/Symfony/Bundle/Command/OpenApiCommand.php
@@ -44,7 +44,7 @@ public function __construct(OpenApiFactoryInterface $openApiFactory, NormalizerI
     /**
      * {@inheritdoc}
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setDescription('Dump the Open API documentation')
@@ -57,7 +57,7 @@ protected function configure()
     /**
      * {@inheritdoc}
      */
-    protected function execute(InputInterface $input, OutputInterface $output)
+    protected function execute(InputInterface $input, OutputInterface $output): int
     {
         // Backwards compatibility
         if (2 === $specVersion = (int) $input->getOption('spec-version')) {
diff --git a/src/Bridge/Symfony/Bundle/Command/SwaggerCommand.php b/src/Bridge/Symfony/Bundle/Command/SwaggerCommand.php
index a6b4b386e6b..62f0eedff07 100644
--- a/src/Bridge/Symfony/Bundle/Command/SwaggerCommand.php
+++ b/src/Bridge/Symfony/Bundle/Command/SwaggerCommand.php
@@ -66,7 +66,7 @@ public function __construct(NormalizerInterface $normalizer, ResourceNameCollect
     /**
      * {@inheritdoc}
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setDescription('Dump the Swagger v2 documentation')
@@ -79,7 +79,7 @@ protected function configure()
     /**
      * {@inheritdoc}
      */
-    protected function execute(InputInterface $input, OutputInterface $output)
+    protected function execute(InputInterface $input, OutputInterface $output): int
     {
         $io = new SymfonyStyle($input, $output);
 
diff --git a/src/Bridge/Symfony/Bundle/DependencyInjection/Configuration.php b/src/Bridge/Symfony/Bundle/DependencyInjection/Configuration.php
index c860f6f02be..d226fe0fbcf 100644
--- a/src/Bridge/Symfony/Bundle/DependencyInjection/Configuration.php
+++ b/src/Bridge/Symfony/Bundle/DependencyInjection/Configuration.php
@@ -48,7 +48,7 @@ final class Configuration implements ConfigurationInterface
     /**
      * {@inheritdoc}
      */
-    public function getConfigTreeBuilder()
+    public function getConfigTreeBuilder(): TreeBuilder
     {
         if (method_exists(TreeBuilder::class, 'getRootNode')) {
             $treeBuilder = new TreeBuilder('api_platform');
diff --git a/src/Bridge/Symfony/Routing/Router.php b/src/Bridge/Symfony/Routing/Router.php
index 51dfe827a2e..0fb6acd0bdf 100644
--- a/src/Bridge/Symfony/Routing/Router.php
+++ b/src/Bridge/Symfony/Routing/Router.php
@@ -46,7 +46,7 @@ public function __construct(RouterInterface $router, int $urlGenerationStrategy
     /**
      * {@inheritdoc}
      */
-    public function setContext(RequestContext $context)
+    public function setContext(RequestContext $context): void
     {
         $this->router->setContext($context);
     }
@@ -54,7 +54,7 @@ public function setContext(RequestContext $context)
     /**
      * {@inheritdoc}
      */
-    public function getContext()
+    public function getContext(): RequestContext
     {
         return $this->router->getContext();
     }
@@ -62,7 +62,7 @@ public function getContext()
     /**
      * {@inheritdoc}
      */
-    public function getRouteCollection()
+    public function getRouteCollection(): \Symfony\Component\Routing\RouteCollection
     {
         return $this->router->getRouteCollection();
     }
@@ -70,11 +70,11 @@ public function getRouteCollection()
     /**
      * {@inheritdoc}
      */
-    public function match($pathInfo)
+    public function match($pathInfo): array
     {
         $baseContext = $this->router->getContext();
         $baseUrl = $baseContext->getBaseUrl();
-        if ($baseUrl === substr($pathInfo, 0, \strlen($baseUrl))) {
+        if (0 === strpos($pathInfo, $baseUrl)) {
             $pathInfo = substr($pathInfo, \strlen($baseUrl));
         }
 
@@ -101,7 +101,7 @@ public function match($pathInfo)
     /**
      * {@inheritdoc}
      */
-    public function generate($name, $parameters = [], $referenceType = null)
+    public function generate($name, $parameters = [], $referenceType = null): string
     {
         return $this->router->generate($name, $parameters, self::CONST_MAP[$referenceType ?? $this->urlGenerationStrategy]);
     }
diff --git a/src/GraphQl/Serializer/ItemNormalizer.php b/src/GraphQl/Serializer/ItemNormalizer.php
index e7c5e6c8a0c..2491f48ce14 100644
--- a/src/GraphQl/Serializer/ItemNormalizer.php
+++ b/src/GraphQl/Serializer/ItemNormalizer.php
@@ -64,6 +64,8 @@ public function supportsNormalization($data, $format = null): bool
      * {@inheritdoc}
      *
      * @throws UnexpectedValueException
+     *
+     * @return array|string|int|float|bool|\ArrayObject|null
      */
     public function normalize($object, $format = null, array $context = [])
     {
@@ -103,6 +105,8 @@ public function supportsDenormalization($data, $type, $format = null): bool
 
     /**
      * {@inheritdoc}
+     *
+     * @return array|bool
      */
     protected function getAllowedAttributes($classOrObject, array $context, $attributesAsString = false)
     {
diff --git a/src/JsonSchema/Command/JsonSchemaGenerateCommand.php b/src/JsonSchema/Command/JsonSchemaGenerateCommand.php
index c492b6f3528..c2ba608fbed 100644
--- a/src/JsonSchema/Command/JsonSchemaGenerateCommand.php
+++ b/src/JsonSchema/Command/JsonSchemaGenerateCommand.php
@@ -47,7 +47,7 @@ public function __construct(SchemaFactoryInterface $schemaFactory, array $format
     /**
      * {@inheritdoc}
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setDescription('Generates the JSON Schema for a resource operation.')
@@ -61,7 +61,7 @@ protected function configure()
     /**
      * {@inheritdoc}
      */
-    protected function execute(InputInterface $input, OutputInterface $output)
+    protected function execute(InputInterface $input, OutputInterface $output): int
     {
         $io = new SymfonyStyle($input, $output);
 
diff --git a/src/Metadata/Property/PropertyNameCollection.php b/src/Metadata/Property/PropertyNameCollection.php
index 785a6ef8919..c938b7d68b6 100644
--- a/src/Metadata/Property/PropertyNameCollection.php
+++ b/src/Metadata/Property/PropertyNameCollection.php
@@ -36,7 +36,7 @@ public function __construct(array $properties = [])
     /**
      * {@inheritdoc}
      */
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new \ArrayIterator($this->properties);
     }
@@ -44,7 +44,7 @@ public function getIterator()
     /**
      * {@inheritdoc}
      */
-    public function count()
+    public function count(): int
     {
         return \count($this->properties);
     }
diff --git a/src/Serializer/AbstractItemNormalizer.php b/src/Serializer/AbstractItemNormalizer.php
index 3e41e9da00d..739913ff4b0 100644
--- a/src/Serializer/AbstractItemNormalizer.php
+++ b/src/Serializer/AbstractItemNormalizer.php
@@ -96,6 +96,8 @@ public function __construct(PropertyNameCollectionFactoryInterface $propertyName
 
     /**
      * {@inheritdoc}
+     *
+     * @return bool
      */
     public function supportsNormalization($data, $format = null)
     {
@@ -118,6 +120,8 @@ public function hasCacheableSupportsMethod(): bool
      * {@inheritdoc}
      *
      * @throws LogicException
+     *
+     * @return array|string|int|float|bool|\ArrayObject|null
      */
     public function normalize($object, $format = null, array $context = [])
     {
@@ -173,6 +177,8 @@ public function normalize($object, $format = null, array $context = [])
 
     /**
      * {@inheritdoc}
+     *
+     * @return bool
      */
     public function supportsDenormalization($data, $type, $format = null)
     {
@@ -260,7 +266,7 @@ public function denormalize($data, $class, $format = null, array $context = [])
      *
      * @internal
      */
-    protected function instantiateObject(array &$data, $class, array &$context, \ReflectionClass $reflectionClass, $allowedAttributes, string $format = null)
+    protected function instantiateObject(array &$data, $class, array &$context, \ReflectionClass $reflectionClass, $allowedAttributes, string $format = null): object
     {
         if (null !== $object = $this->extractObjectToPopulate($class, $context, static::OBJECT_TO_POPULATE)) {
             unset($context[static::OBJECT_TO_POPULATE]);
@@ -344,6 +350,8 @@ protected function createConstructorArgument($parameterData, string $key, \Refle
      * {@inheritdoc}
      *
      * Unused in this context.
+     *
+     * @return array
      */
     protected function extractAttributes($object, $format = null, array $context = [])
     {
@@ -352,6 +360,8 @@ protected function extractAttributes($object, $format = null, array $context = [
 
     /**
      * {@inheritdoc}
+     *
+     * @return array|bool
      */
     protected function getAllowedAttributes($classOrObject, array $context, $attributesAsString = false)
     {
@@ -378,6 +388,8 @@ protected function getAllowedAttributes($classOrObject, array $context, $attribu
 
     /**
      * {@inheritdoc}
+     *
+     * @return bool
      */
     protected function isAllowedAttribute($classOrObject, $attribute, $format = null, array $context = [])
     {
diff --git a/tests/Fixtures/TestBundle/Document/User.php b/tests/Fixtures/TestBundle/Document/User.php
index 4a19447c268..48c13565678 100644
--- a/tests/Fixtures/TestBundle/Document/User.php
+++ b/tests/Fixtures/TestBundle/Document/User.php
@@ -161,7 +161,7 @@ public function getPassword(): ?string
         return null;
     }
 
-    public function getSalt()
+    public function getSalt(): ?string
     {
         return null;
     }
diff --git a/tests/Fixtures/TestBundle/Entity/User.php b/tests/Fixtures/TestBundle/Entity/User.php
index 616b9d4a8d0..be0817142d9 100644
--- a/tests/Fixtures/TestBundle/Entity/User.php
+++ b/tests/Fixtures/TestBundle/Entity/User.php
@@ -164,7 +164,7 @@ public function getPassword(): ?string
         return null;
     }
 
-    public function getSalt()
+    public function getSalt(): ?string
     {
         return null;
     }
diff --git a/tests/Fixtures/TestBundle/Serializer/Denormalizer/SerializableResourceDenormalizer.php b/tests/Fixtures/TestBundle/Serializer/Denormalizer/SerializableResourceDenormalizer.php
index 9d2e8f52df1..295411452bc 100644
--- a/tests/Fixtures/TestBundle/Serializer/Denormalizer/SerializableResourceDenormalizer.php
+++ b/tests/Fixtures/TestBundle/Serializer/Denormalizer/SerializableResourceDenormalizer.php
@@ -37,7 +37,7 @@ public function denormalize($data, $class, $format = null, array $context = [])
     /**
      * {@inheritdoc}
      */
-    public function supportsDenormalization($data, $type, $format = null)
+    public function supportsDenormalization($data, $type, $format = null): bool
     {
         return 'json' === $format && SerializableResource::class === $type && \is_array($data);
     }
diff --git a/tests/Fixtures/TestBundle/Serializer/NameConverter/CustomConverter.php b/tests/Fixtures/TestBundle/Serializer/NameConverter/CustomConverter.php
index 11d8be1806e..74489a5cf9f 100644
--- a/tests/Fixtures/TestBundle/Serializer/NameConverter/CustomConverter.php
+++ b/tests/Fixtures/TestBundle/Serializer/NameConverter/CustomConverter.php
@@ -21,12 +21,12 @@
  */
 class CustomConverter extends CamelCaseToSnakeCaseNameConverter
 {
-    public function normalize($propertyName)
+    public function normalize($propertyName): string
     {
         return 'nameConverted' === $propertyName ? parent::normalize($propertyName) : $propertyName;
     }
 
-    public function denormalize($propertyName)
+    public function denormalize($propertyName): string
     {
         return 'name_converted' === $propertyName ? parent::denormalize($propertyName) : $propertyName;
     }
diff --git a/tests/Fixtures/TestBundle/Serializer/Normalizer/OverrideDocumentationNormalizer.php b/tests/Fixtures/TestBundle/Serializer/Normalizer/OverrideDocumentationNormalizer.php
index 69da8eb7281..30079d7f499 100644
--- a/tests/Fixtures/TestBundle/Serializer/Normalizer/OverrideDocumentationNormalizer.php
+++ b/tests/Fixtures/TestBundle/Serializer/Normalizer/OverrideDocumentationNormalizer.php
@@ -28,6 +28,11 @@ public function __construct(NormalizerInterface $documentationNormalizer)
         $this->documentationNormalizer = $documentationNormalizer;
     }
 
+    /**
+     * @param mixed|null $format
+     *
+     * @return array|string|int|float|bool|\ArrayObject|null
+     */
     public function normalize($object, $format = null, array $context = [])
     {
         $data = $this->documentationNormalizer->normalize($object, $format, $context);
@@ -44,7 +49,7 @@ public function normalize($object, $format = null, array $context = [])
         return $data;
     }
 
-    public function supportsNormalization($data, $format = null)
+    public function supportsNormalization($data, $format = null): bool
     {
         return $this->documentationNormalizer->supportsNormalization($data, $format);
     }