diff --git a/src/AbstractVersion.php b/src/AbstractVersion.php
index eae0bc1..368a65e 100644
--- a/src/AbstractVersion.php
+++ b/src/AbstractVersion.php
@@ -20,8 +20,7 @@ abstract class AbstractVersion implements IVersion
      * which is a value of any type other than a resource.
      * @since 5.4.0
      */
-    #[\ReturnTypeWillChange]
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         return [
             'branch' => $this->getBranch(),
@@ -42,11 +41,11 @@ public function exists(): bool
      * Get the branch string.
      * @return string|null
      */
-    abstract public function getBranch();
+    abstract public function getBranch(): ?string;
 
     /**
      * Get the latest commit ID.
      * @return string|null
      */
-    abstract public function getCommit();
+    abstract public function getCommit(): ?string;
 }
diff --git a/src/FileVersion.php b/src/FileVersion.php
index 54f8d7e..63963f6 100644
--- a/src/FileVersion.php
+++ b/src/FileVersion.php
@@ -33,7 +33,7 @@ class FileVersion extends AbstractVersion
      * deployment script.
      * @param string $file The JSON formatted file containig branch and commit.
      */
-    public function __construct($file)
+    public function __construct(string $file)
     {
         $this->file = $file;
     }
@@ -83,7 +83,7 @@ private function fileContents(): array
      * Get the branch string.
      * @return string|null
      */
-    public function getBranch()
+    public function getBranch(): ?string
     {
         if ($this->fileContents === null) {
             $this->fileContents = $this->fileContents();
@@ -95,7 +95,7 @@ public function getBranch()
      * Get the latest commit ID.
      * @return string|null
      */
-    public function getCommit()
+    public function getCommit(): ?string
     {
         if ($this->fileContents === null) {
             $this->fileContents = $this->fileContents();
diff --git a/src/GitVersion.php b/src/GitVersion.php
index 87c5ade..4a864f3 100644
--- a/src/GitVersion.php
+++ b/src/GitVersion.php
@@ -42,10 +42,10 @@ class GitVersion extends AbstractVersion
      * GitVersion constructor.
      * @param string $appDir The application directory.
      */
-    public function __construct($appDir)
+    public function __construct(string $appDir)
     {
-        $this->dir = $appDir.DIRECTORY_SEPARATOR.'.git';
-        $this->head = $this->dir.DIRECTORY_SEPARATOR.'HEAD';
+        $this->dir = $appDir . DIRECTORY_SEPARATOR . '.git';
+        $this->head = $this->dir . DIRECTORY_SEPARATOR . 'HEAD';
     }
 
     /**
@@ -64,7 +64,7 @@ private function isRepo(): bool
      * Get the branch string.
      * @return string|null
      */
-    public function getBranch()
+    public function getBranch(): ?string
     {
         if ($this->branch === null && $this->isRepo()) {
             $lines = file($this->head);
@@ -81,16 +81,16 @@ public function getBranch()
      * Get the latest commit ID.
      * @return string
      */
-    public function getCommit()
+    public function getCommit(): string
     {
         if ($this->commit === null && $this->isRepo()) {
             /**
              * Build filepath from branch name.
              */
-            $refFile = $this->dir.DIRECTORY_SEPARATOR
-                       .'refs'.DIRECTORY_SEPARATOR
-                       .'heads'.DIRECTORY_SEPARATOR
-                       .$this->getBranch();
+            $refFile = $this->dir . DIRECTORY_SEPARATOR
+                . 'refs' . DIRECTORY_SEPARATOR
+                . 'heads' . DIRECTORY_SEPARATOR
+                . $this->getBranch();
             /**
              * read ref file
              */
diff --git a/src/IVersion.php b/src/IVersion.php
index 0ce87a3..6147c19 100644
--- a/src/IVersion.php
+++ b/src/IVersion.php
@@ -20,13 +20,13 @@ public function exists(): bool;
 
     /**
      * Get the branch string.
-     * @return string
+     * @return string|null
      */
-    public function getBranch();
+    public function getBranch(): ?string;
 
     /**
      * Get the latest commit ID.
-     * @return string
+     * @return string|null
      */
-    public function getCommit();
+    public function getCommit(): ?string;
 }
diff --git a/src/Version.php b/src/Version.php
index a482f4e..050e7aa 100644
--- a/src/Version.php
+++ b/src/Version.php
@@ -36,7 +36,7 @@ public function __construct()
      * @param \kbATeam\Version\IVersion $version
      * @return void
      */
-    public function register(IVersion $version)
+    public function register(IVersion $version): void
     {
         $this->versions[] = $version;
     }
@@ -68,7 +68,7 @@ private function determineVersion(): array
      * Get the branch string.
      * @return string|null
      */
-    public function getBranch()
+    public function getBranch(): ?string
     {
         if ($this->version === null) {
             $this->version = $this->determineVersion();
@@ -80,7 +80,7 @@ public function getBranch()
      * Get the latest commit ID.
      * @return string|null
      */
-    public function getCommit()
+    public function getCommit(): ?string
     {
         if ($this->version === null) {
             $this->version = $this->determineVersion();
diff --git a/tests/FileVersionTest.php b/tests/FileVersionTest.php
index cb562ea..e3f59af 100644
--- a/tests/FileVersionTest.php
+++ b/tests/FileVersionTest.php
@@ -54,7 +54,7 @@ protected function tearDown(): void
      * Test retrieving branch and commit from a JSON encoded file.
      * @return void
      */
-    public function testFileVersionRetrieval()
+    public function testFileVersionRetrieval(): void
     {
         $fileVersion = new FileVersion($this->tempDir.'/commit.json');
         $this->assertInstanceOf(IVersion::class, $fileVersion);
@@ -73,7 +73,7 @@ public function testFileVersionRetrieval()
      * Test retrieving commit from a JSON encoded file.
      * @return void
      */
-    public function testGettingCommit()
+    public function testGettingCommit(): void
     {
         $fileVersion = new FileVersion($this->tempDir.'/commit.json');
         $this->assertSame('9c9e437', $fileVersion->getCommit());
@@ -83,7 +83,7 @@ public function testGettingCommit()
      * Test retrieving branch from a JSON encoded file.
      * @return void
      */
-    public function testGettingBranch()
+    public function testGettingBranch(): void
     {
         $fileVersion = new FileVersion($this->tempDir.'/commit.json');
         $this->assertSame('master', $fileVersion->getBranch());
diff --git a/tests/GitVersionTest.php b/tests/GitVersionTest.php
index 4b6a1dd..7ab4a96 100644
--- a/tests/GitVersionTest.php
+++ b/tests/GitVersionTest.php
@@ -53,7 +53,7 @@ protected function tearDown(): void
      * Test retrieving version information from git.
      * @return void
      */
-    public function testGitVersionRetrieval()
+    public function testGitVersionRetrieval(): void
     {
         $gitVersion = new GitVersion($this->tempDir);
         $this->assertInstanceOf(IVersion::class, $gitVersion);
diff --git a/tests/TempDirTrait.php b/tests/TempDirTrait.php
index bb64bf8..5958f22 100644
--- a/tests/TempDirTrait.php
+++ b/tests/TempDirTrait.php
@@ -18,7 +18,7 @@ trait TempDirTrait
      * @return string
      * @throws \RuntimeException
      */
-    private static function tempdir()
+    private static function tempdir(): string
     {
         ob_start();
         $result = system('mktemp -d', $exitCode);
@@ -38,7 +38,7 @@ private static function tempdir()
      * @throws \RuntimeException
      * @return void
      */
-    private static function rmDir($dir)
+    private static function rmDir(string $dir): void
     {
         if (!is_dir($dir)) {
             throw new \RuntimeException(sprintf("Directory %s doesn't exist", $dir));
diff --git a/tests/VersionTest.php b/tests/VersionTest.php
index 1d96f4f..ca23e78 100644
--- a/tests/VersionTest.php
+++ b/tests/VersionTest.php
@@ -60,7 +60,7 @@ protected function tearDown(): void
      * git repository.
      * @return void
      */
-    public function testRetrievingFileVersion()
+    public function testRetrievingFileVersion(): void
     {
         $version = new Version();
         $this->assertInstanceOf(IVersion::class, $version);
@@ -83,7 +83,7 @@ public function testRetrievingFileVersion()
      * encoded file.
      * @return void
      */
-    public function testRetrievingGitVersion()
+    public function testRetrievingGitVersion(): void
     {
         $version = new Version();
         $this->assertInstanceOf(IVersion::class, $version);