diff --git a/src/Symfony/Component/Cache/Adapter/TagAwareAdapter.php b/src/Symfony/Component/Cache/Adapter/TagAwareAdapter.php
index 539ef1697fbb1..a422bcc0407e3 100644
--- a/src/Symfony/Component/Cache/Adapter/TagAwareAdapter.php
+++ b/src/Symfony/Component/Cache/Adapter/TagAwareAdapter.php
@@ -366,7 +366,7 @@ private function getTagVersions(array $tagsByKey, bool $persistTags): array
             (self::$saveTags)($this->tags, $newTags);
         }
 
-        while ($now > ($this->knownTagVersions[$tag = array_key_first($this->knownTagVersions)][0] ?? \INF)) {
+        while ($now > ($this->knownTagVersions[$tag = array_key_first($this->knownTagVersions) ?? ''][0] ?? \INF)) {
             unset($this->knownTagVersions[$tag]);
         }
 
diff --git a/src/Symfony/Component/Config/Definition/Builder/ArrayNodeDefinition.php b/src/Symfony/Component/Config/Definition/Builder/ArrayNodeDefinition.php
index ec0e12ab6fdf0..e4938352c3234 100644
--- a/src/Symfony/Component/Config/Definition/Builder/ArrayNodeDefinition.php
+++ b/src/Symfony/Component/Config/Definition/Builder/ArrayNodeDefinition.php
@@ -331,7 +331,7 @@ public function normalizeKeys(bool $bool): static
 
     public function append(NodeDefinition $node): static
     {
-        $this->children[$node->name] = $node->setParent($this);
+        $this->children[$node->name ?? ''] = $node->setParent($this);
 
         return $this;
     }
diff --git a/src/Symfony/Component/Console/Input/ArgvInput.php b/src/Symfony/Component/Console/Input/ArgvInput.php
index a33092aee81e1..b5f8666897047 100644
--- a/src/Symfony/Component/Console/Input/ArgvInput.php
+++ b/src/Symfony/Component/Console/Input/ArgvInput.php
@@ -176,7 +176,7 @@ private function parseArgument(string $token): void
         } else {
             $all = $this->definition->getArguments();
             $symfonyCommandName = null;
-            if (($inputArgument = $all[$key = array_key_first($all)] ?? null) && 'command' === $inputArgument->getName()) {
+            if (($inputArgument = $all[$key = array_key_first($all) ?? ''] ?? null) && 'command' === $inputArgument->getName()) {
                 $symfonyCommandName = $this->arguments['command'] ?? null;
                 unset($all[$key]);
             }
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/CheckTypeDeclarationsPass.php b/src/Symfony/Component/DependencyInjection/Compiler/CheckTypeDeclarationsPass.php
index 99652624004a3..e542a2f4970ae 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/CheckTypeDeclarationsPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/CheckTypeDeclarationsPass.php
@@ -79,7 +79,7 @@ public function __construct(bool $autoload = false, array $skippedIds = [])
 
     protected function processValue(mixed $value, bool $isRoot = false): mixed
     {
-        if (isset($this->skippedIds[$this->currentId])) {
+        if (isset($this->skippedIds[$this->currentId ?? ''])) {
             return $value;
         }
 
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/DefinitionErrorExceptionPass.php b/src/Symfony/Component/DependencyInjection/Compiler/DefinitionErrorExceptionPass.php
index 204401cd2c8ee..251f26ef23b4d 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/DefinitionErrorExceptionPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/DefinitionErrorExceptionPass.php
@@ -69,9 +69,9 @@ protected function processValue(mixed $value, bool $isRoot = false): mixed
                 ContainerInterface::RUNTIME_EXCEPTION_ON_INVALID_REFERENCE === $value->getInvalidBehavior()
                 || ContainerInterface::IGNORE_ON_UNINITIALIZED_REFERENCE === $value->getInvalidBehavior()
             ) {
-                $this->sourceReferences[$targetId][$this->currentId] ??= true;
+                $this->sourceReferences[$targetId][$this->currentId ?? ''] ??= true;
             } else {
-                $this->sourceReferences[$targetId][$this->currentId] = false;
+                $this->sourceReferences[$targetId][$this->currentId ?? ''] = false;
             }
 
             return $value;
@@ -81,7 +81,7 @@ protected function processValue(mixed $value, bool $isRoot = false): mixed
             return parent::processValue($value, $isRoot);
         }
 
-        $this->erroredDefinitions[$this->currentId] = $value;
+        $this->erroredDefinitions[$this->currentId ?? ''] = $value;
 
         return parent::processValue($value);
     }
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/InlineServiceDefinitionsPass.php b/src/Symfony/Component/DependencyInjection/Compiler/InlineServiceDefinitionsPass.php
index 9b75fea390dd5..0909cf3f9198a 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/InlineServiceDefinitionsPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/InlineServiceDefinitionsPass.php
@@ -146,7 +146,7 @@ protected function processValue(mixed $value, bool $isRoot = false): mixed
 
         $this->container->log($this, \sprintf('Inlined service "%s" to "%s".', $id, $this->currentId));
         $this->inlinedIds[$id] = $definition->isPublic() || !$definition->isShared();
-        $this->notInlinedIds[$this->currentId] = true;
+        $this->notInlinedIds[$this->currentId ?? ''] = true;
 
         if ($definition->isShared()) {
             return $definition;
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveBindingsPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveBindingsPass.php
index 404709585fe33..9fa6178226ca7 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveBindingsPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveBindingsPass.php
@@ -121,10 +121,10 @@ protected function processValue(mixed $value, bool $isRoot = false): mixed
         foreach ($bindings as $key => $binding) {
             [$bindingValue, $bindingId, $used, $bindingType, $file] = $binding->getValues();
             if ($used) {
-                $this->usedBindings[$bindingId] = true;
-                unset($this->unusedBindings[$bindingId]);
-            } elseif (!isset($this->usedBindings[$bindingId])) {
-                $this->unusedBindings[$bindingId] = [$key, $this->currentId, $bindingType, $file];
+                $this->usedBindings[$bindingId ?? ''] = true;
+                unset($this->unusedBindings[$bindingId ?? '']);
+            } elseif (!isset($this->usedBindings[$bindingId ?? ''])) {
+                $this->unusedBindings[$bindingId ?? ''] = [$key, $this->currentId, $bindingType, $file];
             }
 
             if (preg_match('/^(?:(?:array|bool|float|int|string|iterable|([^ $]++)) )\$/', $key, $m)) {
@@ -263,8 +263,8 @@ private function getBindingValue(BoundArgument $binding): mixed
     {
         [$bindingValue, $bindingId] = $binding->getValues();
 
-        $this->usedBindings[$bindingId] = true;
-        unset($this->unusedBindings[$bindingId]);
+        $this->usedBindings[$bindingId ?? ''] = true;
+        unset($this->unusedBindings[$bindingId ?? '']);
 
         return $bindingValue;
     }
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveHotPathPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveHotPathPass.php
index 705bb837b907c..59d87b3946637 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveHotPathPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveHotPathPass.php
@@ -51,7 +51,7 @@ protected function processValue(mixed $value, bool $isRoot = false): mixed
                 return $value->clearTag('container.hot_path');
             }
 
-            $this->resolvedIds[$this->currentId] = true;
+            $this->resolvedIds[$this->currentId ?? ''] = true;
 
             if (!$value->hasTag('container.hot_path')) {
                 return $value;
diff --git a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
index fdbd290baf88f..bca4356310716 100644
--- a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
+++ b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
@@ -207,7 +207,7 @@ public function registerExtension(ExtensionInterface $extension)
         $this->extensions[$extension->getAlias()] = $extension;
 
         if (false !== $extension->getNamespace()) {
-            $this->extensionsByNs[$extension->getNamespace()] = $extension;
+            $this->extensionsByNs[$extension->getNamespace() ?? ''] = $extension;
         }
     }
 
diff --git a/src/Symfony/Component/Form/Tests/Fixtures/FixedDataTransformer.php b/src/Symfony/Component/Form/Tests/Fixtures/FixedDataTransformer.php
index f3121fc1039da..2a1451c5af5b6 100644
--- a/src/Symfony/Component/Form/Tests/Fixtures/FixedDataTransformer.php
+++ b/src/Symfony/Component/Form/Tests/Fixtures/FixedDataTransformer.php
@@ -25,6 +25,7 @@ public function __construct(array $mapping)
 
     public function transform($value): mixed
     {
+        $value ??= '';
         if (!\array_key_exists($value, $this->mapping)) {
             throw new TransformationFailedException(sprintf('No mapping for value "%s"', $value));
         }
diff --git a/src/Symfony/Component/Form/Tests/Fixtures/FixedFilterListener.php b/src/Symfony/Component/Form/Tests/Fixtures/FixedFilterListener.php
index ece653e87b878..891540c1acae1 100644
--- a/src/Symfony/Component/Form/Tests/Fixtures/FixedFilterListener.php
+++ b/src/Symfony/Component/Form/Tests/Fixtures/FixedFilterListener.php
@@ -50,8 +50,8 @@ public function preSetData(FormEvent $event)
     {
         $data = $event->getData();
 
-        if (isset($this->mapping['preSetData'][$data])) {
-            $event->setData($this->mapping['preSetData'][$data]);
+        if (isset($this->mapping['preSetData'][$data ?? ''])) {
+            $event->setData($this->mapping['preSetData'][$data ?? '']);
         }
     }
 
diff --git a/src/Symfony/Component/HttpFoundation/ResponseHeaderBag.php b/src/Symfony/Component/HttpFoundation/ResponseHeaderBag.php
index 8db8dce4e7516..562f75c9a8ca7 100644
--- a/src/Symfony/Component/HttpFoundation/ResponseHeaderBag.php
+++ b/src/Symfony/Component/HttpFoundation/ResponseHeaderBag.php
@@ -174,7 +174,7 @@ public function getCacheControlDirective(string $key): bool|string|null
      */
     public function setCookie(Cookie $cookie)
     {
-        $this->cookies[$cookie->getDomain()][$cookie->getPath()][$cookie->getName()] = $cookie;
+        $this->cookies[$cookie->getDomain() ?? ''][$cookie->getPath()][$cookie->getName()] = $cookie;
         $this->headerNames['set-cookie'] = 'Set-Cookie';
     }
 
@@ -187,13 +187,13 @@ public function removeCookie(string $name, ?string $path = '/', ?string $domain
     {
         $path ??= '/';
 
-        unset($this->cookies[$domain][$path][$name]);
+        unset($this->cookies[$domain ?? ''][$path][$name]);
 
-        if (empty($this->cookies[$domain][$path])) {
-            unset($this->cookies[$domain][$path]);
+        if (empty($this->cookies[$domain ?? ''][$path])) {
+            unset($this->cookies[$domain ?? ''][$path]);
 
-            if (empty($this->cookies[$domain])) {
-                unset($this->cookies[$domain]);
+            if (empty($this->cookies[$domain ?? ''])) {
+                unset($this->cookies[$domain ?? '']);
             }
         }
 
diff --git a/src/Symfony/Component/Security/Http/Logout/LogoutUrlGenerator.php b/src/Symfony/Component/Security/Http/Logout/LogoutUrlGenerator.php
index 1126a0543ecbd..8550c28c6f99d 100644
--- a/src/Symfony/Component/Security/Http/Logout/LogoutUrlGenerator.php
+++ b/src/Symfony/Component/Security/Http/Logout/LogoutUrlGenerator.php
@@ -146,8 +146,8 @@ private function getListener(?string $key): array
         }
 
         // Fetch from injected current firewall information, if possible
-        if (isset($this->listeners[$this->currentFirewallName])) {
-            return $this->listeners[$this->currentFirewallName];
+        if (isset($this->listeners[$this->currentFirewallName ?? ''])) {
+            return $this->listeners[$this->currentFirewallName ?? ''];
         }
 
         foreach ($this->listeners as $listener) {
diff --git a/src/Symfony/Component/Serializer/Serializer.php b/src/Symfony/Component/Serializer/Serializer.php
index f1caf7d954e5e..0aa24cf579af9 100644
--- a/src/Symfony/Component/Serializer/Serializer.php
+++ b/src/Symfony/Component/Serializer/Serializer.php
@@ -281,8 +281,8 @@ private function getNormalizer(mixed $data, ?string $format, array $context): ?N
             $genericType = '*';
         }
 
-        if (!isset($this->normalizerCache[$format][$type])) {
-            $this->normalizerCache[$format][$type] = [];
+        if (!isset($this->normalizerCache[$format ?? ''][$type])) {
+            $this->normalizerCache[$format ?? ''][$type] = [];
 
             foreach ($this->normalizers as $k => $normalizer) {
                 if (!$normalizer instanceof NormalizerInterface) {
@@ -293,9 +293,9 @@ private function getNormalizer(mixed $data, ?string $format, array $context): ?N
                     trigger_deprecation('symfony/serializer', '6.3', '"%s" should implement "NormalizerInterface::getSupportedTypes(?string $format): array".', $normalizer::class);
 
                     if (!$normalizer instanceof CacheableSupportsMethodInterface || !$normalizer->hasCacheableSupportsMethod()) {
-                        $this->normalizerCache[$format][$type][$k] = false;
+                        $this->normalizerCache[$format ?? ''][$type][$k] = false;
                     } elseif ($normalizer->supportsNormalization($data, $format, $context)) {
-                        $this->normalizerCache[$format][$type][$k] = true;
+                        $this->normalizerCache[$format ?? ''][$type][$k] = true;
                         break;
                     }
 
@@ -313,7 +313,7 @@ private function getNormalizer(mixed $data, ?string $format, array $context): ?N
 
                     if (null === $isCacheable) {
                         unset($supportedTypes['*'], $supportedTypes['object']);
-                    } elseif ($this->normalizerCache[$format][$type][$k] = $isCacheable && $normalizer->supportsNormalization($data, $format, $context)) {
+                    } elseif ($this->normalizerCache[$format ?? ''][$type][$k] = $isCacheable && $normalizer->supportsNormalization($data, $format, $context)) {
                         break 2;
                     }
 
@@ -324,13 +324,13 @@ private function getNormalizer(mixed $data, ?string $format, array $context): ?N
                     continue;
                 }
 
-                if ($this->normalizerCache[$format][$type][$k] ??= $isCacheable && $normalizer->supportsNormalization($data, $format, $context)) {
+                if ($this->normalizerCache[$format ?? ''][$type][$k] ??= $isCacheable && $normalizer->supportsNormalization($data, $format, $context)) {
                     break;
                 }
             }
         }
 
-        foreach ($this->normalizerCache[$format][$type] as $k => $cached) {
+        foreach ($this->normalizerCache[$format ?? ''][$type] as $k => $cached) {
             $normalizer = $this->normalizers[$k];
             if ($cached || $normalizer->supportsNormalization($data, $format, $context)) {
                 return $normalizer;
@@ -350,8 +350,8 @@ private function getNormalizer(mixed $data, ?string $format, array $context): ?N
      */
     private function getDenormalizer(mixed $data, string $class, ?string $format, array $context): ?DenormalizerInterface
     {
-        if (!isset($this->denormalizerCache[$format][$class])) {
-            $this->denormalizerCache[$format][$class] = [];
+        if (!isset($this->denormalizerCache[$format ?? ''][$class])) {
+            $this->denormalizerCache[$format ?? ''][$class] = [];
             $genericType = class_exists($class) || interface_exists($class, false) ? 'object' : '*';
 
             foreach ($this->normalizers as $k => $normalizer) {
@@ -363,9 +363,9 @@ private function getDenormalizer(mixed $data, string $class, ?string $format, ar
                     trigger_deprecation('symfony/serializer', '6.3', '"%s" should implement "DenormalizerInterface::getSupportedTypes(?string $format): array".', $normalizer::class);
 
                     if (!$normalizer instanceof CacheableSupportsMethodInterface || !$normalizer->hasCacheableSupportsMethod()) {
-                        $this->denormalizerCache[$format][$class][$k] = false;
+                        $this->denormalizerCache[$format ?? ''][$class][$k] = false;
                     } elseif ($normalizer->supportsDenormalization(null, $class, $format, $context)) {
-                        $this->denormalizerCache[$format][$class][$k] = true;
+                        $this->denormalizerCache[$format ?? ''][$class][$k] = true;
                         break;
                     }
 
@@ -386,7 +386,7 @@ private function getDenormalizer(mixed $data, string $class, ?string $format, ar
 
                     if (null === $isCacheable) {
                         unset($supportedTypes['*'], $supportedTypes['object']);
-                    } elseif ($this->denormalizerCache[$format][$class][$k] = $isCacheable && $normalizer->supportsDenormalization(null, $class, $format, $context)) {
+                    } elseif ($this->denormalizerCache[$format ?? ''][$class][$k] = $isCacheable && $normalizer->supportsDenormalization(null, $class, $format, $context)) {
                         break 2;
                     }
 
@@ -397,13 +397,13 @@ private function getDenormalizer(mixed $data, string $class, ?string $format, ar
                     continue;
                 }
 
-                if ($this->denormalizerCache[$format][$class][$k] ??= $isCacheable && $normalizer->supportsDenormalization(null, $class, $format, $context)) {
+                if ($this->denormalizerCache[$format ?? ''][$class][$k] ??= $isCacheable && $normalizer->supportsDenormalization(null, $class, $format, $context)) {
                     break;
                 }
             }
         }
 
-        foreach ($this->denormalizerCache[$format][$class] as $k => $cached) {
+        foreach ($this->denormalizerCache[$format ?? ''][$class] as $k => $cached) {
             $normalizer = $this->normalizers[$k];
             if ($cached || $normalizer->supportsDenormalization($data, $class, $format, $context)) {
                 return $normalizer;
diff --git a/src/Symfony/Component/String/Slugger/AsciiSlugger.php b/src/Symfony/Component/String/Slugger/AsciiSlugger.php
index d0c338682ecb3..d4d4e942a68a5 100644
--- a/src/Symfony/Component/String/Slugger/AsciiSlugger.php
+++ b/src/Symfony/Component/String/Slugger/AsciiSlugger.php
@@ -131,8 +131,8 @@ public function slug(string $string, string $separator = '-', ?string $locale =
 
         if (\is_array($this->symbolsMap)) {
             $map = null;
-            if (isset($this->symbolsMap[$locale])) {
-                $map = $this->symbolsMap[$locale];
+            if (isset($this->symbolsMap[$locale ?? ''])) {
+                $map = $this->symbolsMap[$locale ?? ''];
             } else {
                 $parent = self::getParentLocale($locale);
                 if ($parent && isset($this->symbolsMap[$parent])) {
diff --git a/src/Symfony/Component/Translation/DependencyInjection/TranslatorPathsPass.php b/src/Symfony/Component/Translation/DependencyInjection/TranslatorPathsPass.php
index 1756e3c8c58f2..b6112e5b3a523 100644
--- a/src/Symfony/Component/Translation/DependencyInjection/TranslatorPathsPass.php
+++ b/src/Symfony/Component/Translation/DependencyInjection/TranslatorPathsPass.php
@@ -96,10 +96,10 @@ protected function processValue(mixed $value, bool $isRoot = false): mixed
                     $class = $this->definitions[$i]->getClass();
 
                     if (ServiceLocator::class === $class) {
-                        if (!isset($this->controllers[$this->currentId])) {
+                        if (!isset($this->controllers[$this->currentId ?? ''])) {
                             continue;
                         }
-                        foreach ($this->controllers[$this->currentId] as $class => $_) {
+                        foreach ($this->controllers[$this->currentId ?? ''] as $class => $_) {
                             $this->paths[$class] = true;
                         }
                     } else {