diff --git a/.github/workflows/ci-workflow.yml b/.github/workflows/ci-workflow.yml
index 6c43e47..3c5c3b5 100644
--- a/.github/workflows/ci-workflow.yml
+++ b/.github/workflows/ci-workflow.yml
@@ -9,15 +9,15 @@ on:
 jobs:
   code-style:
     name: Code Style & Static Analysis
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: 7.1
+          php-version: 8.4
           extensions: mbstring, pcntl
 
       - name: Validate composer.json and composer.lock
@@ -32,7 +32,7 @@ jobs:
 
   tests:
     name: Test Suite
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       max-parallel: 10
       matrix:
@@ -43,10 +43,13 @@ jobs:
           - 7.4
           - 8.0
           - 8.1
+          - 8.2
+          - 8.3
+          - 8.4
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
diff --git a/src/Pairs.php b/src/Pairs.php
index 53c8f22..f8fd59f 100644
--- a/src/Pairs.php
+++ b/src/Pairs.php
@@ -34,7 +34,7 @@ public function __construct(
         string $queryString,
         bool $decodeKeys = false,
         bool $decodeValues = false,
-        string $separator = null
+        ?string $separator = null
     ) {
 
         $this->queryString = $queryString;
diff --git a/src/QueryString.php b/src/QueryString.php
index e4bbfd8..27f142d 100644
--- a/src/QueryString.php
+++ b/src/QueryString.php
@@ -78,7 +78,7 @@ private static function createFromString(string $string, QueryStringParserInterf
      * @return QueryString
      * @throws \RuntimeException
      */
-    public static function createFromCurrentLocation(QueryStringParserInterface $queryStringParser = null): self
+    public static function createFromCurrentLocation(?QueryStringParserInterface $queryStringParser = null): self
     {
         if (!isset($_SERVER['REQUEST_URI'])) {
             throw new \RuntimeException('$_SERVER[\'REQUEST_URI\'] has not been set.');
@@ -102,7 +102,7 @@ public function withCurrentLocation(): self
      * @throws \InvalidArgumentException
      * @throws \TypeError
      */
-    public static function factory($input = null, QueryStringParserInterface $queryStringParser = null): self
+    public static function factory($input = null, ?QueryStringParserInterface $queryStringParser = null): self
     {
         if (is_array($input)) {
             return self::createFromParams($input);
diff --git a/src/Renderer/ArrayValuesNormalizerRenderer.php b/src/Renderer/ArrayValuesNormalizerRenderer.php
index db0cfc0..5a1d1f2 100644
--- a/src/Renderer/ArrayValuesNormalizerRenderer.php
+++ b/src/Renderer/ArrayValuesNormalizerRenderer.php
@@ -16,12 +16,12 @@ final class ArrayValuesNormalizerRenderer implements QueryStringRendererInterfac
     /**
      * ArrayValuesStringifier constructor.
      */
-    protected function __construct(QueryStringRendererInterface $renderer = null)
+    protected function __construct(?QueryStringRendererInterface $renderer = null)
     {
         $this->renderer = $renderer;
     }
 
-    public static function factory(QueryStringRendererInterface $renderer = null)
+    public static function factory(?QueryStringRendererInterface $renderer = null)
     {
         return new self($renderer ?? NativeRenderer::factory());
     }
diff --git a/src/Renderer/FlatRenderer.php b/src/Renderer/FlatRenderer.php
index f2ffc80..ffad8ab 100644
--- a/src/Renderer/FlatRenderer.php
+++ b/src/Renderer/FlatRenderer.php
@@ -11,12 +11,12 @@ final class FlatRenderer implements QueryStringRendererInterface
      */
     private $renderer;
 
-    protected function __construct(QueryStringRendererInterface $renderer = null)
+    protected function __construct(?QueryStringRendererInterface $renderer = null)
     {
         $this->renderer = $renderer;
     }
 
-    public static function factory(QueryStringRendererInterface $renderer = null)
+    public static function factory(?QueryStringRendererInterface $renderer = null)
     {
         return new self($renderer ?? NativeRenderer::factory());
     }
diff --git a/src/functions.php b/src/functions.php
index 20414f2..fa0a5ee 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -12,7 +12,7 @@
  * @return QueryString
  * @throws \InvalidArgumentException
  */
-function query_string($input = null, QueryStringParserInterface $queryStringParser = null): QueryString
+function query_string($input = null, ?QueryStringParserInterface $queryStringParser = null): QueryString
 {
     return QueryString::factory($input, $queryStringParser);
 }
@@ -21,7 +21,7 @@ function query_string($input = null, QueryStringParserInterface $queryStringPars
  * @param QueryStringRendererInterface|null $renderer
  * @return ArrayValuesNormalizerRenderer
  */
-function withoutNumericIndices(QueryStringRendererInterface $renderer = null): ArrayValuesNormalizerRenderer
+function withoutNumericIndices(?QueryStringRendererInterface $renderer = null): ArrayValuesNormalizerRenderer
 {
     return ArrayValuesNormalizerRenderer::factory($renderer);
 }
@@ -30,7 +30,7 @@ function withoutNumericIndices(QueryStringRendererInterface $renderer = null): A
  * @param QueryStringRendererInterface|null $renderer
  * @return FlatRenderer
  */
-function flat(QueryStringRendererInterface $renderer = null): FlatRenderer
+function flat(?QueryStringRendererInterface $renderer = null): FlatRenderer
 {
     return FlatRenderer::factory($renderer);
 }
@@ -42,7 +42,7 @@ function flat(QueryStringRendererInterface $renderer = null): FlatRenderer
  * @param string|null $separator
  * @return Pairs
  */
-function pairs(string $queryString, bool $decodeKeys = false, bool $decodeValues = false, string $separator = null): Pairs
+function pairs(string $queryString, bool $decodeKeys = false, bool $decodeValues = false, ?string $separator = null): Pairs
 {
     return new Pairs($queryString, $decodeKeys, $decodeValues, $separator);
 }