diff --git a/src/Rules/DoNotUseImplicitVariableInConfigurationFilesRule.php b/src/Rules/DoNotUseImplicitVariableInConfigurationFilesRule.php
new file mode 100644
index 0000000..a65ebc0
--- /dev/null
+++ b/src/Rules/DoNotUseImplicitVariableInConfigurationFilesRule.php
@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ssch\Typo3PhpstanRules\Rules;
+
+use PhpParser\Node;
+use PhpParser\Node\Expr\Variable;
+use PHPStan\Analyser\Scope;
+use Ssch\Typo3PhpstanRules\FileResolver;
+use Symplify\PHPStanRules\Rules\AbstractSymplifyRule;
+use Symplify\RuleDocGenerator\ValueObject\CodeSample\CodeSample;
+use Symplify\RuleDocGenerator\ValueObject\RuleDefinition;
+
+final class DoNotUseImplicitVariableInConfigurationFilesRule extends AbstractSymplifyRule
+{
+    /**
+     * @var string
+     */
+    private const MESSAGE = 'Do not use implicit variable "$%s" in file "%s"';
+
+    /**
+     * @var string[]
+     */
+    private const DISALLOWED_VARIABLES = ['_EXTKEY', '_EXTCONF'];
+
+    private FileResolver $fileResolver;
+
+    public function __construct(FileResolver $fileResolver)
+    {
+        $this->fileResolver = $fileResolver;
+    }
+
+    public function getRuleDefinition(): RuleDefinition
+    {
+        return new RuleDefinition('Do not use $_EXTKEY and $_EXTCONF in ext_localconf.php or ext_tables.php', [
+            new CodeSample(
+                <<<'CODE_SAMPLE'
+$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['t3lib/class.t3lib_pagerenderer.php']['render-preProcess'][$_EXTKEY] = \Prefix\MyExtension\PageRendererHooks::class . '->renderPreProcess';
+CODE_SAMPLE
+                ,
+                <<<'CODE_SAMPLE'
+$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['t3lib/class.t3lib_pagerenderer.php']['render-preProcess']['my_extension_key'] = \Prefix\MyExtension\PageRendererHooks::class . '->renderPreProcess';
+CODE_SAMPLE
+            ),
+        ]);
+    }
+
+    public function getNodeTypes(): array
+    {
+        return [Node\Expr\Variable::class];
+    }
+
+    public function process(Node $node, Scope $scope): array
+    {
+        if (! $node instanceof Variable) {
+            return [];
+        }
+
+        if (! $this->fileResolver->isConfigurationFile($scope)) {
+            return [];
+        }
+
+        if (! is_string($node->name)) {
+            return [];
+        }
+
+        if (! in_array($node->name, self::DISALLOWED_VARIABLES, true)) {
+            return [];
+        }
+
+        $message = sprintf(self::MESSAGE, $node->name, $scope->getFile());
+
+        return [$message];
+    }
+}
diff --git a/src/Rules/DoNotUseMagicConstantFileAndDirInConfigurationFilesRule.php b/src/Rules/DoNotUseMagicConstantFileAndDirInConfigurationFilesRule.php
index 345eb2c..57f5a7a 100644
--- a/src/Rules/DoNotUseMagicConstantFileAndDirInConfigurationFilesRule.php
+++ b/src/Rules/DoNotUseMagicConstantFileAndDirInConfigurationFilesRule.php
@@ -41,7 +41,7 @@ public function getRuleDefinition(): RuleDefinition
 __DIR__ . '/Resources/Private/Templates/Template.html;
 CODE_SAMPLE
                 ,
-                \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::class . '::extPath(\'my_extension\') . \'/Resources/Private/Templates/Template.html;'
+                \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::class . "::extPath('my_extension') . '/Resources/Private/Templates/Template.html;"
             ),
         ]);
     }
diff --git a/src/Rules/MissingDefaultValueForTypedPropertyRule.php b/src/Rules/MissingDefaultValueForTypedPropertyRule.php
index feb2b03..3f90a77 100644
--- a/src/Rules/MissingDefaultValueForTypedPropertyRule.php
+++ b/src/Rules/MissingDefaultValueForTypedPropertyRule.php
@@ -29,23 +29,27 @@ public function __construct(EntityClassDetector $entityClassDetector)
 
     public function getRuleDefinition(): RuleDefinition
     {
-        return new RuleDefinition(self::ERROR_MESSAGE, [
-            new CodeSample(
-                <<<'CODE_SAMPLE'
+        return new RuleDefinition(
+            'Missing default value for property "property" in class "MissingDefaultValueForTypedProperty"',
+            [
+                new CodeSample(
+                    <<<'CODE_SAMPLE'
 final class MissingDefaultValueForTypedProperty extends \TYPO3\CMS\Extbase\DomainObject\AbstractEntity
 {
     protected string $property;
 }
 CODE_SAMPLE
-                ,
-                <<<'CODE_SAMPLE'
+                    ,
+                    <<<'CODE_SAMPLE'
 final class MissingDefaultValueForTypedProperty extends \TYPO3\CMS\Extbase\DomainObject\AbstractEntity
 {
     protected string $property = '';
 }
 CODE_SAMPLE
-            ),
-        ]);
+                ),
+
+            ]
+        );
     }
 
     /**
diff --git a/src/Rules/MissingVarAnnotationForPropertyInEntityClassRule.php b/src/Rules/MissingVarAnnotationForPropertyInEntityClassRule.php
index 92bd4f3..52b8046 100644
--- a/src/Rules/MissingVarAnnotationForPropertyInEntityClassRule.php
+++ b/src/Rules/MissingVarAnnotationForPropertyInEntityClassRule.php
@@ -37,7 +37,7 @@ public function __construct(EntityClassDetector $entityClassDetector)
 
     public function getRuleDefinition(): RuleDefinition
     {
-        return new RuleDefinition(self::ERROR_MESSAGE, [
+        return new RuleDefinition('Missing var annotation for property "property" in class "MissingDocblock"', [
             new CodeSample(
                 <<<'CODE_SAMPLE'
 final class MissingDocblock extends \TYPO3\CMS\Extbase\DomainObject\AbstractEntity
diff --git a/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/DoNotUseImplicitVariableInConfigurationFilesRuleTest.php b/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/DoNotUseImplicitVariableInConfigurationFilesRuleTest.php
new file mode 100644
index 0000000..bfad050
--- /dev/null
+++ b/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/DoNotUseImplicitVariableInConfigurationFilesRuleTest.php
@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ssch\Typo3PhpstanRules\Tests\Rules\DoNotUseMagicConstantFileAndDirInConfigurationFilesRule;
+
+use Iterator;
+use PHPStan\Rules\Rule;
+use Ssch\Typo3PhpstanRules\Rules\DoNotUseImplicitVariableInConfigurationFilesRule;
+use Symplify\PHPStanExtensions\Testing\AbstractServiceAwareRuleTestCase;
+
+/**
+ * @extends AbstractServiceAwareRuleTestCase<DoNotUseImplicitVariableInConfigurationFilesRule>
+ */
+final class DoNotUseImplicitVariableInConfigurationFilesRuleTest extends AbstractServiceAwareRuleTestCase
+{
+    /**
+     * @dataProvider provideData()
+     *
+     * @param array<string|int> $expectedErrorMessagesWithLines
+     */
+    public function testRule(string $filePath, array $expectedErrorMessagesWithLines): void
+    {
+        $this->analyse([$filePath], $expectedErrorMessagesWithLines);
+    }
+
+    public function provideData(): Iterator
+    {
+        $invalidFile = __DIR__ . '/MyExtension/ext_localconf.php';
+
+        yield [
+            $invalidFile,
+            [
+                [sprintf('Do not use implicit variable "$_EXTCONF" in file "%s"', $invalidFile), 3],
+                [sprintf('Do not use implicit variable "$_EXTKEY" in file "%s"', $invalidFile), 5],
+            ],
+        ];
+
+        yield [__DIR__ . '/MySecondExtension/ext_localconf.php', []];
+    }
+
+    protected function getRule(): Rule
+    {
+        return $this->getRuleFromConfig(
+            DoNotUseImplicitVariableInConfigurationFilesRule::class,
+            __DIR__ . '/config/configured_rule.neon'
+        );
+    }
+}
diff --git a/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/MyExtension/ext_localconf.php b/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/MyExtension/ext_localconf.php
new file mode 100644
index 0000000..e2e2e60
--- /dev/null
+++ b/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/MyExtension/ext_localconf.php
@@ -0,0 +1,5 @@
+<?php
+
+$foo = $_EXTCONF['foo'];
+
+$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['t3lib/class.t3lib_pagerenderer.php']['render-preProcess'][$_EXTKEY] = \Prefix\MyExtension\PageRendererHooks::class . '->renderPreProcess';
diff --git a/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/MySecondExtension/ext_localconf.php b/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/MySecondExtension/ext_localconf.php
new file mode 100644
index 0000000..50cce95
--- /dev/null
+++ b/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/MySecondExtension/ext_localconf.php
@@ -0,0 +1,3 @@
+<?php
+
+
diff --git a/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/config/configured_rule.neon b/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/config/configured_rule.neon
new file mode 100644
index 0000000..eabbe5c
--- /dev/null
+++ b/tests/Rules/DoNotUseImplicitVariableInConfigurationFilesRule/config/configured_rule.neon
@@ -0,0 +1,7 @@
+includes:
+    - ../../../config/included_services.neon
+
+services:
+    -
+        class: Ssch\Typo3PhpstanRules\Rules\DoNotUseImplicitVariableInConfigurationFilesRule
+        tags: [phpstan.rules.rule]