diff --git a/Build/Scripts/phpstan.sh b/Build/Scripts/phpstan.sh
new file mode 100755
index 0000000..7a95b00
--- /dev/null
+++ b/Build/Scripts/phpstan.sh
@@ -0,0 +1,13 @@
+#!/bin/bash
+
+THIS_SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null && pwd)"
+cd "$THIS_SCRIPT_DIR" || exit 1
+cd ../../ || exit 1
+CORE_ROOT="${PWD}"
+
+Build/Scripts/runTests.sh -s composerInstall
+
+Build/Scripts/runTests.sh -s phpstan
+
+Build/Scripts/runTests.sh -s clean
+Build/Scripts/additionalTests.sh -s clean
diff --git a/Build/phpstan/phpstan-baseline.neon b/Build/phpstan/phpstan-baseline.neon
new file mode 100644
index 0000000..1339890
--- /dev/null
+++ b/Build/phpstan/phpstan-baseline.neon
@@ -0,0 +1,2 @@
+parameters:
+	ignoreErrors:
\ No newline at end of file
diff --git a/Build/phpstan/phpstan-constants.php b/Build/phpstan/phpstan-constants.php
new file mode 100644
index 0000000..0bb0280
--- /dev/null
+++ b/Build/phpstan/phpstan-constants.php
@@ -0,0 +1,4 @@
+<?php
+
+// testing-framework defines this, used in various tests.
+define('ORIGINAL_ROOT', dirname(__FILE__, 2) . '/');
diff --git a/Build/phpstan/phpstan.local.neon b/Build/phpstan/phpstan.local.neon
new file mode 100644
index 0000000..d89a1f1
--- /dev/null
+++ b/Build/phpstan/phpstan.local.neon
@@ -0,0 +1,2 @@
+includes:
+  - phpstan.neon
diff --git a/Build/phpstan/phpstan.neon b/Build/phpstan/phpstan.neon
new file mode 100644
index 0000000..e7d5a93
--- /dev/null
+++ b/Build/phpstan/phpstan.neon
@@ -0,0 +1,21 @@
+includes:
+  - phpstan-baseline.neon
+  - ../vendor/friendsoftypo3/phpstan-typo3/extension.neon
+
+parameters:
+  # Use local .cache dir instead of /tmp
+  tmpDir: ../.cache/phpstan
+
+  level: 6
+
+  bootstrapFiles:
+    - phpstan-constants.php
+
+  paths:
+    - ../../
+
+  excludePaths:
+    # we do not check required extensions
+    - ../../Build/*
+    # ext_emconf.php get the $_EXTKEY set from outside. We'll ignore all of them
+    - ../ext_emconf.php
diff --git a/Classes/Controller/FileController.php b/Classes/Controller/FileController.php
index bf759ab..d45ebaa 100644
--- a/Classes/Controller/FileController.php
+++ b/Classes/Controller/FileController.php
@@ -115,6 +115,10 @@ public function convertFileAction(ServerRequestInterface $request): ResponseInte
         return $moduleTemplate->renderResponse('File/ConvertFile');
     }
 
+    /**
+     * @return array<string|array<array<string, string>>>
+     * @throws PropagateResponseException
+     */
     protected function prepareExtensions(ServerRequestInterface $request, bool $selected = true): array
     {
         $extensions = $this->extensionService->getLocalExtensions();
@@ -133,6 +137,10 @@ protected function prepareExtensions(ServerRequestInterface $request, bool $sele
         return [$extensions, $selectedExtension, $selectedExtensionKey];
     }
 
+    /**
+     * @return array<string|array<array<string, string>>>
+     * @throws PropagateResponseException
+     */
     protected function prepareFiles(ServerRequestInterface $request, string $extension, bool $selected = true): array
     {
         $files = $this->extensionService->getFilesOfExtension($extension);
@@ -152,6 +160,7 @@ protected function prepareFiles(ServerRequestInterface $request, string $extensi
     }
 
     /**
+     * @param array<string, mixed> $extensions
      * @throws PropagateResponseException
      */
     protected function getSelectedExtension(ServerRequestInterface $request, array $extensions): string
@@ -164,6 +173,7 @@ protected function getSelectedExtension(ServerRequestInterface $request, array $
     }
 
     /**
+     * @param array<string, mixed> $files
      * @throws PropagateResponseException
      */
     protected function getSelectedFile(ServerRequestInterface $request, array $files): string
@@ -175,6 +185,9 @@ protected function getSelectedFile(ServerRequestInterface $request, array $files
         return $selectedFile;
     }
 
+    /**
+     * @param array<string, mixed> $values
+     */
     protected function isArgumentSetAndAvailable(ServerRequestInterface $request, array $values, string $key): ?string
     {
         $formFieldValue = (string)($request->getParsedBody()[$key] ?? $request->getQueryParams()[$key] ?? '');
diff --git a/Classes/File/Converter.php b/Classes/File/Converter.php
index 5b41883..46f69b5 100644
--- a/Classes/File/Converter.php
+++ b/Classes/File/Converter.php
@@ -105,8 +105,7 @@ protected function checkLanguageFilename(string $xmlFile): string
 
     /**
      * @param string $languageFile Absolute reference to the base locallang file
-     *
-     * @return array
+     * @return string[]
      */
     protected function getAvailableTranslations(string $languageFile): array
     {
@@ -198,10 +197,9 @@ protected function getOriginalFileName(string $filename, string $langKey): strin
 
     /**
      * Reads/Requires locallang files and returns raw $LOCAL_LANG array
-     *
      * @param string $languageFile Absolute reference to the ll-XML locallang file.
-     *
-     * @return array LOCAL_LANG array from ll-XML file (with all possible sub-files for languages included)
+     * @return array<string, array<string, string>> LOCAL_LANG array from ll-XML file
+     *  (with all possible sub-files for languages included)
      */
     protected function getCombinedTranslationFileContent(string $languageFile): array
     {
@@ -211,7 +209,7 @@ protected function getCombinedTranslationFileContent(string $languageFile): arra
             $includedLanguages = array_keys($ll['data']);
 
             foreach ($includedLanguages as $langKey) {
-                /** @var $parser LocallangXmlParser */
+                /** @var LocallangXmlParser $parser */
                 $parser = GeneralUtility::makeInstance(LocallangXmlParser::class);
                 $localLangContent = $parser->getParsedData($languageFile, $langKey);
                 unset($parser);
@@ -219,7 +217,7 @@ protected function getCombinedTranslationFileContent(string $languageFile): arra
             }
         } else {
             require($languageFile);
-            $includedLanguages = isset($LOCAL_LANG) ? array_keys($LOCAL_LANG) : [];
+            $includedLanguages = array_keys($LOCAL_LANG);
         }
 
         if (empty($includedLanguages)) {
@@ -238,7 +236,7 @@ protected function getCombinedTranslationFileContent(string $languageFile): arra
      * @param string $namespacePrefix The tag-prefix resolve, e.g. a namespace like "T3:"
      * @param bool $reportDocTag If set, the document tag will be set in the key "_DOCUMENT_TAG" of the output array
      *
-     * @return array|string If the parsing had errors, a string with the error message is returned.
+     * @return array<string, mixed>|string If the parsing had errors, a string with the error message is returned.
      *         Otherwise, an array with the content.
      *
      * @see GeneralUtility::array2xml(),GeneralUtility::xml2arrayProcess()
diff --git a/Classes/File/TransUnit.php b/Classes/File/TransUnit.php
index f0b2e73..25b3ebc 100644
--- a/Classes/File/TransUnit.php
+++ b/Classes/File/TransUnit.php
@@ -21,6 +21,12 @@
 
     protected string $target;
 
+    /**
+     * @param string|array<string, string> $data
+     * @param string $key
+     * @param string $langKey
+     * @param array<string, array<string, string>> $LOCAL_LANG
+     */
     public function __construct(
         string|array $data,
         protected string $key,
diff --git a/Classes/Localization/Parser/LocallangXmlParser.php b/Classes/Localization/Parser/LocallangXmlParser.php
index 1c88389..c99b593 100644
--- a/Classes/Localization/Parser/LocallangXmlParser.php
+++ b/Classes/Localization/Parser/LocallangXmlParser.php
@@ -29,6 +29,7 @@ class LocallangXmlParser extends AbstractXmlParser
 {
     /**
      * Associative array of "filename => parsed data" pairs.
+     * @var array<string, array<string, mixed>> $parsedTargetFiles
      */
     protected array $parsedTargetFiles = [];
 
@@ -38,7 +39,7 @@ class LocallangXmlParser extends AbstractXmlParser
      * @param string $sourcePath Source file path
      * @param string $languageKey Language key
      *
-     * @return array
+     * @return array<string, array<string, string>>
      *
      * @throws FileNotFoundException
      * @throws InvalidXmlFileException
@@ -71,6 +72,7 @@ public function getParsedData($sourcePath, $languageKey): array
 
     /**
      * Parse the given language key tag
+     * @return array<string, array<string, string>>
      */
     protected function getParsedDataForElement(\SimpleXMLElement $bodyOfFileTag, string $element): array
     {
@@ -97,6 +99,7 @@ protected function getParsedDataForElement(\SimpleXMLElement $bodyOfFileTag, str
 
     /**
      * Returns array representation of XLIFF data, starting from a root node.
+     * @return array<string, array<string, string>>
      */
     protected function doParsingFromRoot(\SimpleXMLElement $root): array
     {
@@ -105,6 +108,7 @@ protected function doParsingFromRoot(\SimpleXMLElement $root): array
 
     /**
      * Returns array representation of XLIFF data, starting from a root node.
+     * @return array<string, array<string, string>>
      */
     protected function doParsingTargetFromRoot(\SimpleXMLElement $root): array
     {
@@ -113,6 +117,7 @@ protected function doParsingTargetFromRoot(\SimpleXMLElement $root): array
 
     /**
      * Returns array representation of XLIFF data, starting from a root node.
+     * @return array<string, array<string, string>>
      */
     protected function doParsingFromRootForElement(\SimpleXMLElement $root, string $element): array
     {
@@ -153,6 +158,7 @@ protected function doParsingFromRootForElement(\SimpleXMLElement $root, string $
      * Returns parsed representation of XML file.
      *
      * Parses XML if it wasn't done before. Caches parsed data.
+     * @return array<string, array<string, string>>
      */
     protected function getParsedTargetData(string $path): array
     {
@@ -164,6 +170,7 @@ protected function getParsedTargetData(string $path): array
 
     /**
      * Reads and parses XML file and returns internal representation of data.
+     * @return array<string, array<string, string>>
      */
     protected function parseXmlTargetFile(string $targetPath): array
     {
diff --git a/Classes/Service/ExtensionService.php b/Classes/Service/ExtensionService.php
index e07c51e..b419b43 100644
--- a/Classes/Service/ExtensionService.php
+++ b/Classes/Service/ExtensionService.php
@@ -27,6 +27,9 @@ public function __construct(
         protected Converter $converter,
     ) {}
 
+    /**
+     * @return array<array<string, mixed>>
+     */
     public function getLocalExtensions(): array
     {
         $availableExtensions = $this->listUtility->getAvailableExtensions();
@@ -52,6 +55,7 @@ function (array $extension) {
 
     /**
      * Gather files for given extension key that need to be converted
+     * @return array<string, array<string, string>>
      */
     public function getFilesOfExtension(string $extensionKey): array
     {
@@ -82,6 +86,10 @@ protected function isLanguageFile(string $filePath): bool
         return str_contains($filePath, 'Resources/Private/Language/');
     }
 
+    /**
+     * @param array<string, array<string, string>> $files
+     * @return array<string, string|bool>
+     */
     public function convertLanguageFile(string $selectedExtension, string $selectedFile, array $files): array
     {
         $wasConvertedPreviously = false;
diff --git a/composer.json b/composer.json
index 13a2137..2905930 100644
--- a/composer.json
+++ b/composer.json
@@ -35,7 +35,11 @@
 		"psr/http-message": "^1.1 || ^2.0"
 	},
 	"require-dev": {
-		"friendsofphp/php-cs-fixer": "^3.57.1",
+		"friendsofphp/php-cs-fixer": "^3.64.0",
+		"friendsoftypo3/phpstan-typo3": "^0.9.0",
+		"phpstan/phpdoc-parser": "^1.30.0",
+		"phpstan/phpstan": "^1.12.5",
+		"phpunit/phpunit": "^11.0.3",
 		"typo3/testing-framework": "dev-main"
 	},
 	"minimum-stability": "dev",