diff --git a/.github/workflows/phpunit.yml b/.github/workflows/phpunit.yml
index 7462b7b..31b617c 100644
--- a/.github/workflows/phpunit.yml
+++ b/.github/workflows/phpunit.yml
@@ -8,12 +8,12 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-versions: [7.4, 8.0, 8.2, 8.3]
+        php-versions: [7.4, 8.0, 8.2, 8.3, 8.4]
     name: PHP ${{ matrix.php-versions }}
 
     steps:
     - name: Checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
     
     - name: Install PHP
       uses: shivammathur/setup-php@v2
@@ -33,7 +33,7 @@ jobs:
 
     - name: Cache Composer packages
       id: composer-cache
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: vendor
         key: ${{ runner.os }}-php-${{ hashFiles('**/composer.lock') }}
diff --git a/.github/workflows/phpunit_php5.yml b/.github/workflows/phpunit_php5.yml
index e8b353e..8d6eb04 100644
--- a/.github/workflows/phpunit_php5.yml
+++ b/.github/workflows/phpunit_php5.yml
@@ -13,7 +13,7 @@ jobs:
 
     steps:
     - name: Checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
     
     - name: Install PHP
       uses: shivammathur/setup-php@v2
diff --git a/src/AbstractJsonSerializeObjData.php b/src/AbstractJsonSerializeObjData.php
index b82801c..7b4ad87 100644
--- a/src/AbstractJsonSerializeObjData.php
+++ b/src/AbstractJsonSerializeObjData.php
@@ -69,7 +69,9 @@ final protected static function objectToJsonData($obj, $flags = 0, $objParents =
             if ($includeProps !==  true && !in_array($propName, $includeProps)) {
                 continue;
             }
-            $prop->setAccessible(true);
+            if (PHP_VERSION_ID < 80100) {
+                $prop->setAccessible(true);
+            }
             $propValue = $prop->getValue($obj);
             $result[$propName] = self::valueToJsonData($propValue, $flags, $objParents);
         }
@@ -237,7 +239,9 @@ final protected static function fillObjFromValue($value, $obj, $flags = 0, $map
             } else {
                 $reflect = new ReflectionObject($obj);
                 foreach ($reflect->getProperties() as $prop) {
-                    $prop->setAccessible(true);
+                    if (PHP_VERSION_ID < 80100) {
+                        $prop->setAccessible(true);
+                    }
                     $propName = $prop->getName();
                     if ($map !== null) {
                         $map->setCurrent($propName, $current);
diff --git a/src/JsonSerialize.php b/src/JsonSerialize.php
index 67428bb..8da562f 100644
--- a/src/JsonSerialize.php
+++ b/src/JsonSerialize.php
@@ -114,7 +114,7 @@ public static function unserializeWithMap($json, JsonUnserializeMap $map, $depth
     public static function unserializeToObj($json, $obj, $depth = 512, $flags = 0)
     {
         if (is_object($obj)) {
-        } elseif (is_string($obj) && class_exists($obj)) {
+        } elseif (is_string($obj) && class_exists($obj)) { // @phpstan-ignore function.alreadyNarrowedType
             $obj = self::getObjFromClass($obj);
         } else {
             throw new Exception('invalid obj param');
@@ -210,7 +210,7 @@ protected static function convertString($string)
         if ($use_mb) {
             $encoding = mb_detect_encoding($string, null, true);
             if ($encoding) {
-                return mb_convert_encoding($string, 'UTF-8', $encoding);
+                return (string) mb_convert_encoding($string, 'UTF-8', $encoding);
             } else {
                 return mb_convert_encoding($string, 'UTF-8', 'UTF-8');
             }
diff --git a/src/JsonUnserializeMap.php b/src/JsonUnserializeMap.php
index 6b162a5..09b5fec 100644
--- a/src/JsonUnserializeMap.php
+++ b/src/JsonUnserializeMap.php
@@ -46,11 +46,11 @@ class JsonUnserializeMap
     /**
      * Class constructor
      *
-     * @param array<string, string> $map values map
+     * @param array<string,string> $map values map
      */
     public function __construct($map = [])
     {
-        if (!is_array($map)) {
+        if (!is_array($map)) { // @phpstan-ignore function.alreadyNarrowedType
             throw new Exception('map must be an array');
         }
         $this->map = new MapItem();
diff --git a/tests/StdClassTest.php b/tests/StdClassTest.php
index 9ef38c4..8295ea1 100644
--- a/tests/StdClassTest.php
+++ b/tests/StdClassTest.php
@@ -51,7 +51,7 @@ public function testStdClass()
         $serializedValue = JsonSerialize::serialize($value);
         $this->assertTrue(is_string($serializedValue), 'Value is string');
         $unserializedValue = JsonSerialize::unserialize($serializedValue);
-        $this->assertSame($unserializedValue->c->e, null, 'Test stdClass object recursion'); /** @phpstan-ignore-line */
+        $this->assertSame($unserializedValue->c->e, null, 'Test stdClass object recursion');
 
         $obj = new stdClass();
         $obj->a = 1;
diff --git a/tests/phpstan.neon b/tests/phpstan.neon
index e27cfd9..20d7634 100644
--- a/tests/phpstan.neon
+++ b/tests/phpstan.neon
@@ -1,5 +1,5 @@
 parameters:
-    level: 9
+    level: 8
     paths:
         - ..
     excludePaths: