diff --git a/Zend/tests/enum/json_encode.phpt b/Zend/tests/enum/json_encode.phpt
index 016ca6107fee..12ed0d3d445a 100644
--- a/Zend/tests/enum/json_encode.phpt
+++ b/Zend/tests/enum/json_encode.phpt
@@ -18,7 +18,7 @@ enum StringFoo: string {
 enum CustomFoo implements JsonSerializable {
     case Bar;
 
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         return 'Custom ' . $this->name;
     }
 }
diff --git a/ext/json/json.stub.php b/ext/json/json.stub.php
index 5b0d72cef27e..f819af71f9b5 100644
--- a/ext/json/json.stub.php
+++ b/ext/json/json.stub.php
@@ -12,8 +12,8 @@ function json_last_error_msg(): string {}
 
 interface JsonSerializable
 {
-    /** @return mixed */
-    public function jsonSerialize();
+    /** @tentative-return-type */
+    public function jsonSerialize(): mixed;
 }
 
 class JsonException extends Exception
diff --git a/ext/json/json_arginfo.h b/ext/json/json_arginfo.h
index ee8954c3ed37..447ea568a4fd 100644
--- a/ext/json/json_arginfo.h
+++ b/ext/json/json_arginfo.h
@@ -1,5 +1,5 @@
 /* This is a generated file, edit the .stub.php file instead.
- * Stub hash: 6dcef13e46c3dfc2546d3ccc38da07e6c009bbe5 */
+ * Stub hash: d6577c7d27442643c86e51d462f51dd6e677d509 */
 
 ZEND_BEGIN_ARG_WITH_RETURN_TYPE_MASK_EX(arginfo_json_encode, 0, 1, MAY_BE_STRING|MAY_BE_FALSE)
 	ZEND_ARG_TYPE_INFO(0, value, IS_MIXED, 0)
@@ -20,7 +20,7 @@ ZEND_END_ARG_INFO()
 ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_json_last_error_msg, 0, 0, IS_STRING, 0)
 ZEND_END_ARG_INFO()
 
-ZEND_BEGIN_ARG_INFO_EX(arginfo_class_JsonSerializable_jsonSerialize, 0, 0, 0)
+ZEND_BEGIN_ARG_WITH_TENTATIVE_RETURN_TYPE_INFO_EX(arginfo_class_JsonSerializable_jsonSerialize, 0, 0, IS_MIXED, 0)
 ZEND_END_ARG_INFO()
 
 
diff --git a/ext/json/tests/bug61978.phpt b/ext/json/tests/bug61978.phpt
index 8853bdbbd280..283432f8773b 100644
--- a/ext/json/tests/bug61978.phpt
+++ b/ext/json/tests/bug61978.phpt
@@ -17,7 +17,7 @@ class JsonTest2 implements JsonSerializable {
     public function __construct() {
         $this->test = '123';
     }
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         return array(
             'test' => $this->test,
             'me'   => $this
diff --git a/ext/json/tests/bug66025.phpt b/ext/json/tests/bug66025.phpt
index 8f12fa40ca0f..628adefc6511 100644
--- a/ext/json/tests/bug66025.phpt
+++ b/ext/json/tests/bug66025.phpt
@@ -4,7 +4,7 @@ Bug #66025 (Indent wrong when json_encode() called from jsonSerialize function)
 <?php
 
 class Foo implements JsonSerializable {
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         return json_encode([1], JSON_PRETTY_PRINT);
     }
 }
diff --git a/ext/json/tests/bug68992.phpt b/ext/json/tests/bug68992.phpt
index f618f66ee43f..e83d8a84a3d9 100644
--- a/ext/json/tests/bug68992.phpt
+++ b/ext/json/tests/bug68992.phpt
@@ -4,7 +4,7 @@ Bug #68992 (json_encode stacks exceptions thrown by JsonSerializable classes)
 <?php
 
 class MyClass implements JsonSerializable {
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         throw new Exception('Not implemented!');
     }
 }
diff --git a/ext/json/tests/bug71835.phpt b/ext/json/tests/bug71835.phpt
index a1c885046e3e..341d9c6cfc62 100644
--- a/ext/json/tests/bug71835.phpt
+++ b/ext/json/tests/bug71835.phpt
@@ -3,7 +3,7 @@ Bug #71835 (json_encode sometimes incorrectly detects recursion with JsonSeriali
 --FILE--
 <?php
 class SomeClass implements JsonSerializable {
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         return [get_object_vars($this)];
     }
 }
@@ -12,7 +12,7 @@ $arr = [$class];
 var_dump(json_encode($arr));
 
 class SomeClass2 implements JsonSerializable {
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         return [(array)$this];
     }
 }
diff --git a/ext/json/tests/bug72069.phpt b/ext/json/tests/bug72069.phpt
index a37aa38ea176..a9c762dbf1ac 100644
--- a/ext/json/tests/bug72069.phpt
+++ b/ext/json/tests/bug72069.phpt
@@ -8,7 +8,7 @@ var_dump($result);
 
 class A implements \JsonSerializable
 {
-    function jsonSerialize()
+    function jsonSerialize(): mixed
     {
         return ['end' => json_decode('', true)];
     }
diff --git a/ext/json/tests/bug73113.phpt b/ext/json/tests/bug73113.phpt
index 36d78ffdd4d1..d22b0c23f71b 100644
--- a/ext/json/tests/bug73113.phpt
+++ b/ext/json/tests/bug73113.phpt
@@ -6,7 +6,7 @@ Also test that the custom exception is not wrapped by ext/json
 
 class JsonSerializableObject implements \JsonSerializable
 {
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         throw new \Exception('This error is expected');
     }
diff --git a/ext/json/tests/bug77843.phpt b/ext/json/tests/bug77843.phpt
index c525285bf5ab..2709612659c6 100644
--- a/ext/json/tests/bug77843.phpt
+++ b/ext/json/tests/bug77843.phpt
@@ -5,7 +5,7 @@ Bug #77843: Use after free with json serializer
 
 class X implements JsonSerializable {
     public $prop = "value";
-    public function jsonSerialize() {
+    public function jsonSerialize(): mixed {
         global $arr;
         unset($arr[0]);
         var_dump($this);
diff --git a/ext/json/tests/serialize.phpt b/ext/json/tests/serialize.phpt
index 288e3f673376..31b36221ef9e 100644
--- a/ext/json/tests/serialize.phpt
+++ b/ext/json/tests/serialize.phpt
@@ -17,7 +17,7 @@ class NonSerializingTest
 
 class SerializingTest extends NonSerializingTest implements JsonSerializable
 {
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         return $this->data;
     }
@@ -25,7 +25,7 @@ class SerializingTest extends NonSerializingTest implements JsonSerializable
 
 class ValueSerializingTest extends SerializingTest
 {
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         return array_values(is_array($this->data) ? $this->data : get_object_vars($this->data));
     }
@@ -33,7 +33,7 @@ class ValueSerializingTest extends SerializingTest
 
 class SelfSerializingTest extends SerializingTest
 {
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         return $this;
     }
diff --git a/ext/standard/tests/class_object/get_object_vars_variation_004.phpt b/ext/standard/tests/class_object/get_object_vars_variation_004.phpt
index 902149ea0ac4..7c21b903adfc 100644
Binary files a/ext/standard/tests/class_object/get_object_vars_variation_004.phpt and b/ext/standard/tests/class_object/get_object_vars_variation_004.phpt differ