diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index d286d28..d79bc54 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -30,4 +30,4 @@ jobs:
       os: >-
         ['ubuntu-latest', 'windows-latest']
       php: >-
-        ['8.1', '8.2', '8.3']
+        ['8.1', '8.2', '8.3', '8.4']
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 64fa5db..f32e4c0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@ Change Log
 ## 0.1.3 Under development
 
 - Enh #87: Refactor `SemverUtil::class` stability normalization logic (@terabytesoftw)
+- Bug #104: Add support for PHP 8.4 (@terabytesoftw)
 
 ## 0.1.2 June 10, 2024
 
diff --git a/composer.json b/composer.json
index be34835..1ee332c 100644
--- a/composer.json
+++ b/composer.json
@@ -25,7 +25,7 @@
         "php-forge/support": "^0.1",
         "phpunit/phpunit": "^10.5",
         "symplify/easy-coding-standard": "^12.1",
-        "vimeo/psalm": "^5.20"
+        "vimeo/psalm": "^5.26.1|^6.4.1"
     },
     "config": {
         "preferred-install": {
diff --git a/src/Asset/AssetManagerFinder.php b/src/Asset/AssetManagerFinder.php
index 251f048..6f0c9dc 100644
--- a/src/Asset/AssetManagerFinder.php
+++ b/src/Asset/AssetManagerFinder.php
@@ -52,7 +52,7 @@ public function addManager(AssetManagerInterface $manager): void
      * @throws RuntimeException When the asset manager does not exist
      * @throws RuntimeException When the asset manager is not found
      */
-    public function findManager(string $manager = null): AssetManagerInterface
+    public function findManager(string|null $manager = null): AssetManagerInterface
     {
         if (null !== $manager) {
             if (isset($this->managers[$manager])) {
diff --git a/src/Config/ConfigBuilder.php b/src/Config/ConfigBuilder.php
index abcbb00..b3a6903 100644
--- a/src/Config/ConfigBuilder.php
+++ b/src/Config/ConfigBuilder.php
@@ -31,7 +31,7 @@ abstract class ConfigBuilder
      * @param array $defaults The default values.
      * @param IOInterface|null $io The composer input/output.
      */
-    public static function build(Composer $composer, array $defaults = [], IOInterface $io = null): Config
+    public static function build(Composer $composer, array $defaults = [], IOInterface|null $io = null): Config
     {
         $config = self::getConfigBase($composer, $io);
 
@@ -44,7 +44,7 @@ public static function build(Composer $composer, array $defaults = [], IOInterfa
      * @param Composer $composer The composer.
      * @param IOInterface|null $io The composer input/output.
      */
-    private static function getConfigBase(Composer $composer, IOInterface $io = null): array
+    private static function getConfigBase(Composer $composer, IOInterface|null $io = null): array
     {
         $globalPackageConfig = self::getGlobalConfig($composer, 'composer', $io);
         $globalConfig = self::getGlobalConfig($composer, 'config', $io);
@@ -63,7 +63,7 @@ private static function getConfigBase(Composer $composer, IOInterface $io = null
      * @param string $filename The filename.
      * @param IOInterface|null $io The composer input/output.
      */
-    private static function getGlobalConfig(Composer $composer, string $filename, IOInterface $io = null): array
+    private static function getGlobalConfig(Composer $composer, string $filename, IOInterface|null $io = null): array
     {
         $home = self::getComposerHome($composer);
         $file = new JsonFile($home . '/' . $filename . '.json');
diff --git a/src/Converter/SemverConverter.php b/src/Converter/SemverConverter.php
index 2b59c30..cbb70bc 100644
--- a/src/Converter/SemverConverter.php
+++ b/src/Converter/SemverConverter.php
@@ -20,7 +20,7 @@
  */
 final class SemverConverter implements VersionConverterInterface
 {
-    public function convertVersion(string $version = null): string
+    public function convertVersion(string|null $version = null): string
     {
         if (\in_array($version, [null, '', 'latest'], true)) {
             return ('latest' === $version ? 'default || ' : '') . '*';
diff --git a/src/Converter/VersionConverterInterface.php b/src/Converter/VersionConverterInterface.php
index 4c19bba..2577f48 100644
--- a/src/Converter/VersionConverterInterface.php
+++ b/src/Converter/VersionConverterInterface.php
@@ -27,5 +27,5 @@ interface VersionConverterInterface
      *
      * @return string The composer version
      */
-    public function convertVersion(string $version = null): string;
+    public function convertVersion(string|null $version = null): string;
 }
diff --git a/src/Fallback/AssetFallback.php b/src/Fallback/AssetFallback.php
index 9103a41..591a2d4 100644
--- a/src/Fallback/AssetFallback.php
+++ b/src/Fallback/AssetFallback.php
@@ -31,7 +31,7 @@ public function __construct(
         protected IOInterface $io,
         protected Config $config,
         protected string $path,
-        Filesystem $fs = null
+        Filesystem|null $fs = null
     ) {
         $this->fs = $fs ?: new Filesystem();
     }
diff --git a/src/Fallback/ComposerFallback.php b/src/Fallback/ComposerFallback.php
index 8412f11..aa4a654 100644
--- a/src/Fallback/ComposerFallback.php
+++ b/src/Fallback/ComposerFallback.php
@@ -48,7 +48,7 @@ public function __construct(
         protected IOInterface $io,
         protected Config $config,
         protected InputInterface $input,
-        Filesystem $fs = null,
+        Filesystem|null $fs = null,
         protected Installer|null $installer = null
     ) {
         $this->fs = $fs ?: new Filesystem();