diff --git a/src/Symfony/Bundle/FrameworkBundle/Test/TestContainer.php b/src/Symfony/Bundle/FrameworkBundle/Test/TestContainer.php
index ee97ad0a695ca..3494d090d653a 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Test/TestContainer.php
+++ b/src/Symfony/Bundle/FrameworkBundle/Test/TestContainer.php
@@ -11,6 +11,7 @@
 
 namespace Symfony\Bundle\FrameworkBundle\Test;
 
+use Psr\Container\ContainerInterface;
 use Symfony\Component\DependencyInjection\Container;
 use Symfony\Component\DependencyInjection\ParameterBag\ParameterBagInterface;
 use Symfony\Component\HttpKernel\KernelInterface;
@@ -137,7 +138,7 @@ public function getRemovedIds(): array
         return $this->getPublicContainer()->getRemovedIds();
     }
 
-    private function getPublicContainer()
+    private function getPublicContainer(): Container
     {
         if (null === $container = $this->kernel->getContainer()) {
             throw new \LogicException('Cannot access the container on a non-booted kernel. Did you forget to boot it?');
@@ -146,7 +147,7 @@ private function getPublicContainer()
         return $container;
     }
 
-    private function getPrivateContainer()
+    private function getPrivateContainer(): ContainerInterface
     {
         return $this->getPublicContainer()->get($this->privateServicesLocatorId);
     }
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/AbstractRecursivePass.php b/src/Symfony/Component/DependencyInjection/Compiler/AbstractRecursivePass.php
index eab5a6ec87b9f..f1206a7a109b4 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/AbstractRecursivePass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/AbstractRecursivePass.php
@@ -192,7 +192,7 @@ protected function getReflectionMethod(Definition $definition, $method)
         return $r;
     }
 
-    private function getExpressionLanguage()
+    private function getExpressionLanguage(): ExpressionLanguage
     {
         if (null === $this->expressionLanguage) {
             if (!class_exists(ExpressionLanguage::class)) {
@@ -200,7 +200,7 @@ private function getExpressionLanguage()
             }
 
             $providers = $this->container->getExpressionLanguageProviders();
-            $this->expressionLanguage = new ExpressionLanguage(null, $providers, function ($arg) {
+            $this->expressionLanguage = new ExpressionLanguage(null, $providers, function (string $arg): string {
                 if ('""' === substr_replace($arg, '', 1, -1)) {
                     $id = stripcslashes(substr($arg, 1, -1));
                     $this->inExpression = true;
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/AutowirePass.php b/src/Symfony/Component/DependencyInjection/Compiler/AutowirePass.php
index 4aed93b3608c9..8d46fd6311035 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/AutowirePass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/AutowirePass.php
@@ -81,6 +81,9 @@ protected function processValue($value, $isRoot = false)
         }
     }
 
+    /**
+     * @return mixed
+     */
     private function doProcessValue($value, bool $isRoot = false)
     {
         if ($value instanceof TypedReference) {
@@ -371,7 +374,7 @@ private function set(string $type, string $id)
         $this->ambiguousServiceTypes[$type][] = $id;
     }
 
-    private function createTypeNotFoundMessageCallback(TypedReference $reference, string $label)
+    private function createTypeNotFoundMessageCallback(TypedReference $reference, string $label): callable
     {
         if (null === $this->typesClone->container) {
             $this->typesClone->container = new ContainerBuilder($this->container->getParameterBag());
@@ -386,7 +389,7 @@ private function createTypeNotFoundMessageCallback(TypedReference $reference, st
         })->bindTo($this->typesClone);
     }
 
-    private function createTypeNotFoundMessage(TypedReference $reference, string $label, string $currentId)
+    private function createTypeNotFoundMessage(TypedReference $reference, string $label, string $currentId): string
     {
         if (!$r = $this->container->getReflectionClass($type = $reference->getType(), false)) {
             // either $type does not exist or a parent class does not exist
@@ -420,7 +423,7 @@ private function createTypeNotFoundMessage(TypedReference $reference, string $la
         return $message;
     }
 
-    private function createTypeAlternatives(ContainerBuilder $container, TypedReference $reference)
+    private function createTypeAlternatives(ContainerBuilder $container, TypedReference $reference): string
     {
         // try suggesting available aliases first
         if ($message = $this->getAliasesSuggestionForType($container, $type = $reference->getType())) {
@@ -444,7 +447,7 @@ private function createTypeAlternatives(ContainerBuilder $container, TypedRefere
         return sprintf(' You should maybe alias this %s to %s.', class_exists($type, false) ? 'class' : 'interface', $message);
     }
 
-    private function getAliasesSuggestionForType(ContainerBuilder $container, string $type)
+    private function getAliasesSuggestionForType(ContainerBuilder $container, string $type): ?string
     {
         $aliases = [];
         foreach (class_parents($type) + class_implements($type) as $parent) {
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/RegisterEnvVarProcessorsPass.php b/src/Symfony/Component/DependencyInjection/Compiler/RegisterEnvVarProcessorsPass.php
index 852e7ca44aae3..a9a133be2a737 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/RegisterEnvVarProcessorsPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/RegisterEnvVarProcessorsPass.php
@@ -60,7 +60,7 @@ public function process(ContainerBuilder $container)
         }
     }
 
-    private static function validateProvidedTypes($types, $class)
+    private static function validateProvidedTypes(string $types, string $class): array
     {
         $types = explode('|', $types);
 
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveBindingsPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveBindingsPass.php
index 23fe4cababb6b..58581a7e91879 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveBindingsPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveBindingsPass.php
@@ -205,6 +205,9 @@ protected function processValue($value, $isRoot = false)
         return parent::processValue($value, $isRoot);
     }
 
+    /**
+     * @return mixed
+     */
     private function getBindingValue(BoundArgument $binding)
     {
         list($bindingValue, $bindingId) = $binding->getValues();
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveHotPathPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveHotPathPass.php
index 5def580589f0f..bfdd91c417f97 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveHotPathPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveHotPathPass.php
@@ -26,7 +26,7 @@ class ResolveHotPathPass extends AbstractRecursivePass
     private $tagName;
     private $resolvedIds = [];
 
-    public function __construct($tagName = 'container.hot_path')
+    public function __construct(string $tagName = 'container.hot_path')
     {
         $this->tagName = $tagName;
     }
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveInstanceofConditionalsPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveInstanceofConditionalsPass.php
index f165c9e03fafc..be8f8e7fd4d86 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveInstanceofConditionalsPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveInstanceofConditionalsPass.php
@@ -44,7 +44,7 @@ public function process(ContainerBuilder $container)
         }
     }
 
-    private function processDefinition(ContainerBuilder $container, string $id, Definition $definition)
+    private function processDefinition(ContainerBuilder $container, string $id, Definition $definition): Definition
     {
         $instanceofConditionals = $definition->getInstanceofConditionals();
         $autoconfiguredInstanceof = $definition->isAutoconfigured() ? $container->getAutoconfiguredInstanceof() : [];
@@ -144,7 +144,7 @@ private function processDefinition(ContainerBuilder $container, string $id, Defi
         return $definition;
     }
 
-    private function mergeConditionals(array $autoconfiguredInstanceof, array $instanceofConditionals, ContainerBuilder $container)
+    private function mergeConditionals(array $autoconfiguredInstanceof, array $instanceofConditionals, ContainerBuilder $container): array
     {
         // make each value an array of ChildDefinition
         $conditionals = array_map(function ($childDef) { return [$childDef]; }, $autoconfiguredInstanceof);
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveInvalidReferencesPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveInvalidReferencesPass.php
index d7ebe69eb8858..a0686554e1fd3 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveInvalidReferencesPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveInvalidReferencesPass.php
@@ -51,6 +51,8 @@ public function process(ContainerBuilder $container)
     /**
      * Processes arguments to determine invalid references.
      *
+     * @return mixed
+     *
      * @throws RuntimeException When an invalid reference is found
      */
     private function processValue($value, int $rootLevel = 0, int $level = 0)
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ServiceLocatorTagPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ServiceLocatorTagPass.php
index dea70174b8841..a4b740b9e0e0d 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ServiceLocatorTagPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ServiceLocatorTagPass.php
@@ -88,9 +88,8 @@ protected function processValue($value, $isRoot = false)
 
     /**
      * @param Reference[] $refMap
-     * @param string|null $callerId
      */
-    public static function register(ContainerBuilder $container, array $refMap, $callerId = null): Reference
+    public static function register(ContainerBuilder $container, array $refMap, string $callerId = null): Reference
     {
         foreach ($refMap as $id => $ref) {
             if (!$ref instanceof Reference) {
diff --git a/src/Symfony/Component/DependencyInjection/Container.php b/src/Symfony/Component/DependencyInjection/Container.php
index d48ac026cf385..00bacd2ac6ded 100644
--- a/src/Symfony/Component/DependencyInjection/Container.php
+++ b/src/Symfony/Component/DependencyInjection/Container.php
@@ -422,9 +422,14 @@ protected function getEnv($name)
     }
 
     /**
+     * @param string|false $registry
+     * @param string|bool  $load
+     *
+     * @return mixed
+     *
      * @internal
      */
-    final protected function getService($registry, $id, $method, $load)
+    final protected function getService($registry, string $id, ?string $method, $load)
     {
         if ('service_container' === $id) {
             return $this;
diff --git a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
index 4de09c315bee0..19f41d99653cc 100644
--- a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
+++ b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
@@ -1236,7 +1236,7 @@ private function doResolveServices($value, array &$inlineServices = [], bool $is
 
                     yield $k => $this->resolveServices($v);
                 }
-            }, function () use ($value) {
+            }, function () use ($value): int {
                 $count = 0;
                 foreach ($value->getValues() as $v) {
                     foreach (self::getServiceConditionals($v) as $s) {
@@ -1641,7 +1641,7 @@ private function shareService(Definition $definition, $service, ?string $id, arr
         }
     }
 
-    private function getExpressionLanguage()
+    private function getExpressionLanguage(): ExpressionLanguage
     {
         if (null === $this->expressionLanguage) {
             if (!class_exists('Symfony\Component\ExpressionLanguage\ExpressionLanguage')) {
@@ -1653,7 +1653,7 @@ private function getExpressionLanguage()
         return $this->expressionLanguage;
     }
 
-    private function inVendors(string $path)
+    private function inVendors(string $path): bool
     {
         if (null === $this->vendors) {
             $resource = new ComposerResource();
diff --git a/src/Symfony/Component/DependencyInjection/Dumper/GraphvizDumper.php b/src/Symfony/Component/DependencyInjection/Dumper/GraphvizDumper.php
index 08c3e306fb12d..c21dd91c5159e 100644
--- a/src/Symfony/Component/DependencyInjection/Dumper/GraphvizDumper.php
+++ b/src/Symfony/Component/DependencyInjection/Dumper/GraphvizDumper.php
@@ -183,7 +183,7 @@ private function findNodes(): array
         return $nodes;
     }
 
-    private function cloneContainer()
+    private function cloneContainer(): ContainerBuilder
     {
         $parameterBag = new ParameterBag($this->container->getParameterBag()->all());
 
diff --git a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
index 22c5a7ef19b21..26b1842063b97 100644
--- a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
+++ b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
@@ -684,7 +684,7 @@ private function addServiceMethodCalls(Definition $definition, string $variableN
         return $calls;
     }
 
-    private function addServiceProperties(Definition $definition, string $variableName = 'instance')
+    private function addServiceProperties(Definition $definition, string $variableName = 'instance'): string
     {
         $code = '';
         foreach ($definition->getProperties() as $name => $value) {
@@ -967,7 +967,7 @@ private function addServices(array &$services = null): string
         return $publicServices.$privateServices;
     }
 
-    private function generateServiceFiles(array $services)
+    private function generateServiceFiles(array $services): iterable
     {
         $definitions = $this->container->getDefinitions();
         ksort($definitions);
@@ -993,7 +993,7 @@ private function generateServiceFiles(array $services)
         }
     }
 
-    private function addNewInstance(Definition $definition, string $return = '', string $id = null)
+    private function addNewInstance(Definition $definition, string $return = '', string $id = null): string
     {
         $tail = $return ? ";\n" : '';
 
@@ -1914,7 +1914,7 @@ private function getNextVariableName(): string
         }
     }
 
-    private function getExpressionLanguage()
+    private function getExpressionLanguage(): ExpressionLanguage
     {
         if (null === $this->expressionLanguage) {
             if (!class_exists('Symfony\Component\ExpressionLanguage\ExpressionLanguage')) {
@@ -1941,7 +1941,7 @@ private function getExpressionLanguage()
         return $this->expressionLanguage;
     }
 
-    private function isHotPath(Definition $definition)
+    private function isHotPath(Definition $definition): bool
     {
         return $this->hotPathTag && $definition->hasTag($this->hotPathTag) && !$definition->isDeprecated();
     }
@@ -1965,6 +1965,9 @@ private function isSingleUsePrivateNode(ServiceReferenceGraphNode $node): bool
         return 1 === \count($ids);
     }
 
+    /**
+     * @return mixed
+     */
     private function export($value)
     {
         if (null !== $this->targetDirRegex && \is_string($value) && preg_match($this->targetDirRegex, $value, $matches, PREG_OFFSET_CAPTURE)) {
@@ -1988,6 +1991,9 @@ private function export($value)
         return $this->doExport($value, true);
     }
 
+    /**
+     * @return mixed
+     */
     private function doExport($value, bool $resolveEnv = false)
     {
         $shouldCacheValue = $resolveEnv && \is_string($value);
diff --git a/src/Symfony/Component/DependencyInjection/Dumper/XmlDumper.php b/src/Symfony/Component/DependencyInjection/Dumper/XmlDumper.php
index e78173dffad22..43a22f65ee933 100644
--- a/src/Symfony/Component/DependencyInjection/Dumper/XmlDumper.php
+++ b/src/Symfony/Component/DependencyInjection/Dumper/XmlDumper.php
@@ -338,11 +338,9 @@ private function escape(array $arguments): array
      *
      * @param mixed $value Value to convert
      *
-     * @return string
-     *
      * @throws RuntimeException When trying to dump object or resource
      */
-    public static function phpToXml($value)
+    public static function phpToXml($value): string
     {
         switch (true) {
             case null === $value:
diff --git a/src/Symfony/Component/DependencyInjection/Dumper/YamlDumper.php b/src/Symfony/Component/DependencyInjection/Dumper/YamlDumper.php
index 41fd762747673..24f55969b5047 100644
--- a/src/Symfony/Component/DependencyInjection/Dumper/YamlDumper.php
+++ b/src/Symfony/Component/DependencyInjection/Dumper/YamlDumper.php
@@ -201,6 +201,8 @@ private function addParameters(): string
      * Dumps callable to YAML format.
      *
      * @param mixed $callable
+     *
+     * @return mixed
      */
     private function dumpCallable($callable)
     {
@@ -299,7 +301,7 @@ private function getParameterCall(string $id): string
         return sprintf('%%%s%%', $id);
     }
 
-    private function getExpressionCall(string $expression)
+    private function getExpressionCall(string $expression): string
     {
         return sprintf('@=%s', $expression);
     }
diff --git a/src/Symfony/Component/DependencyInjection/Extension/Extension.php b/src/Symfony/Component/DependencyInjection/Extension/Extension.php
index 925775aa567c3..23b154b1a6877 100644
--- a/src/Symfony/Component/DependencyInjection/Extension/Extension.php
+++ b/src/Symfony/Component/DependencyInjection/Extension/Extension.php
@@ -111,7 +111,7 @@ final protected function processConfiguration(ConfigurationInterface $configurat
     /**
      * @internal
      */
-    final public function getProcessedConfigs()
+    final public function getProcessedConfigs(): array
     {
         try {
             return $this->processedConfigs;
diff --git a/src/Symfony/Component/DependencyInjection/Loader/Configurator/PrototypeConfigurator.php b/src/Symfony/Component/DependencyInjection/Loader/Configurator/PrototypeConfigurator.php
index 119f2f252e177..3cd56e0f19a48 100644
--- a/src/Symfony/Component/DependencyInjection/Loader/Configurator/PrototypeConfigurator.php
+++ b/src/Symfony/Component/DependencyInjection/Loader/Configurator/PrototypeConfigurator.php
@@ -75,7 +75,7 @@ public function __destruct()
      *
      * @return $this
      */
-    final public function exclude($excludes)
+    final public function exclude($excludes): self
     {
         $this->excludes = (array) $excludes;
 
diff --git a/src/Symfony/Component/DependencyInjection/Loader/FileLoader.php b/src/Symfony/Component/DependencyInjection/Loader/FileLoader.php
index cb204164857c3..307e71d1e6835 100644
--- a/src/Symfony/Component/DependencyInjection/Loader/FileLoader.php
+++ b/src/Symfony/Component/DependencyInjection/Loader/FileLoader.php
@@ -102,7 +102,7 @@ protected function setDefinition($id, Definition $definition)
         }
     }
 
-    private function findClasses(string $namespace, string $pattern, array $excludePatterns)
+    private function findClasses(string $namespace, string $pattern, array $excludePatterns): array
     {
         $parameterBag = $this->container->getParameterBag();
 
diff --git a/src/Symfony/Component/DependencyInjection/Loader/IniFileLoader.php b/src/Symfony/Component/DependencyInjection/Loader/IniFileLoader.php
index bb9f12a7d9ec8..40137b18fd7a3 100644
--- a/src/Symfony/Component/DependencyInjection/Loader/IniFileLoader.php
+++ b/src/Symfony/Component/DependencyInjection/Loader/IniFileLoader.php
@@ -66,6 +66,8 @@ public function supports($resource, $type = null)
      * Note that the following features are not supported:
      *  * strings with escaped quotes are not supported "foo\"bar";
      *  * string concatenation ("foo" "bar").
+     *
+     * @return mixed
      */
     private function phpize(string $value)
     {
diff --git a/src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php b/src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php
index 50b90a81aabad..3fc723b686793 100644
--- a/src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php
+++ b/src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php
@@ -435,7 +435,7 @@ private function processAnonymousServices(\DOMDocument $xml, string $file)
         }
     }
 
-    private function getArgumentsAsPhp(\DOMElement $node, string $name, string $file, bool $isChildDefinition = false)
+    private function getArgumentsAsPhp(\DOMElement $node, string $name, string $file, bool $isChildDefinition = false): array
     {
         $arguments = [];
         foreach ($this->getChildren($node, $name) as $arg) {
diff --git a/src/Symfony/Component/DependencyInjection/ServiceLocator.php b/src/Symfony/Component/DependencyInjection/ServiceLocator.php
index d7bbecfa276d1..e379f3b607cbb 100644
--- a/src/Symfony/Component/DependencyInjection/ServiceLocator.php
+++ b/src/Symfony/Component/DependencyInjection/ServiceLocator.php
@@ -64,8 +64,10 @@ public function __invoke($id)
 
     /**
      * @internal
+     *
+     * @return static
      */
-    public function withContext($externalId, Container $container)
+    public function withContext(string $externalId, Container $container)
     {
         $locator = clone $this;
         $locator->externalId = $externalId;
@@ -127,7 +129,7 @@ private function createCircularReferenceException(string $id, array $path): Cont
         return new ServiceCircularReferenceException($id, $path);
     }
 
-    private function formatAlternatives(array $alternatives = null, string $separator = 'and')
+    private function formatAlternatives(array $alternatives = null, string $separator = 'and'): string
     {
         $format = '"%s"%s';
         if (null === $alternatives) {
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveParameterPlaceHoldersPassTest.php b/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveParameterPlaceHoldersPassTest.php
index a81634d9d9e7f..b5f02de9405e2 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveParameterPlaceHoldersPassTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveParameterPlaceHoldersPassTest.php
@@ -71,7 +71,7 @@ public function testBindingsShouldBeResolved()
         $this->assertSame($this->container->getParameterBag()->resolveValue('%env(BAZ)%'), $boundValue);
     }
 
-    private function createContainerBuilder()
+    private function createContainerBuilder(): ContainerBuilder
     {
         $containerBuilder = new ContainerBuilder();
 
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Dumper/YamlDumperTest.php b/src/Symfony/Component/DependencyInjection/Tests/Dumper/YamlDumperTest.php
index 56512e89c1481..9b1ff3245f118 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Dumper/YamlDumperTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Dumper/YamlDumperTest.php
@@ -110,7 +110,7 @@ public function testTaggedArguments()
         $this->assertStringEqualsFile(self::$fixturesPath.'/yaml/services_with_tagged_argument.yml', $dumper->dump());
     }
 
-    private function assertEqualYamlStructure($expected, $yaml, $message = '')
+    private function assertEqualYamlStructure(string $expected, string $yaml, string $message = '')
     {
         $parser = new Parser();