Fix incorrect serialisation. Thanks to @Westie for reporting the issue

lib/Checkout/JsonSerializer.php

@@ -27,11 +27,16 @@ private function normalize(array $props)
             return !is_null($value);
         });
         foreach ($array as $key => $value) {
-            // customization
             if ($value instanceof DateTime) {
                 $array[$key] = CheckoutUtils::formatDate($value);
+            } elseif (is_array($value)) {
+                $array[$key] = $this->normalize($value);
             } elseif (is_object($value)) {
-                $array[$key] = $this->normalize(get_object_vars($value));
+                if (version_compare(PHP_VERSION, '7.1.0') >= 0 && is_iterable($value)) {
+                    $array[$key] = $this->normalize(iterator_to_array($value));
+                } else {
+                    $array[$key] = $this->normalize(get_object_vars($value));
+                }
             }
             $array = $this->applyKeysTransformations($array, $key);
         }

phpstan.neon.dist

@@ -5,3 +5,6 @@ parameters:
     paths:
         - lib
         - test
+
+    ignoreErrors:
+        - '#Return type mixed of method .*::getIterator\(\) is not covariant with tentative return type Traversable of method IteratorAggregate::getIterator\(\).#'

test/Checkout/Tests/IterableObject.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Checkout\Tests;
+
+use ArrayIterator;
+use IteratorAggregate;
+
+class IterableObject implements IteratorAggregate
+{
+    private $items;
+
+    public function __construct(array $items)
+    {
+        $this->items = $items;
+    }
+
+    public function getIterator()
+    {
+        return new ArrayIterator($this->items);
+    }
+}

test/Checkout/Tests/JsonSerializerTest.php

@@ -0,0 +1,121 @@
+<?php
+
+namespace Checkout\Tests;
+
+use Checkout\CheckoutUtils;
+use Checkout\JsonSerializer;
+use DateTime;
+use PHPUnit\Framework\TestCase;
+
+class JsonSerializerTest extends TestCase
+{
+    public function testSerializeWithSimpleArray()
+    {
+        $serializer = new JsonSerializer();
+        $input = [
+            'name' => 'John Doe',
+            'age' => 30,
+            'null_value' => null
+        ];
+        $expected = json_encode([
+            'name' => 'John Doe',
+            'age' => 30
+        ]);
+
+        $this->assertEquals($expected, $serializer->serialize($input));
+    }
+
+    public function testSerializeWithDateTime()
+    {
+        $serializer = new JsonSerializer();
+        $date = new DateTime('2023-08-22');
+        $input = [
+            'name' => 'John Doe',
+            'created_at' => $date
+        ];
+        $expected = json_encode([
+            'name' => 'John Doe',
+            'created_at' => CheckoutUtils::formatDate($date)
+        ]);
+
+        $this->assertEquals($expected, $serializer->serialize($input));
+    }
+
+    public function testSerializeWithNestedArray()
+    {
+        $serializer = new JsonSerializer();
+        $input = [
+            'user' => [
+                'name' => 'John Doe',
+                'address' => [
+                    'city' => 'New York',
+                    'zip' => '10001',
+                    'null_value' => null
+                ]
+            ]
+        ];
+        $expected = json_encode([
+            'user' => [
+                'name' => 'John Doe',
+                'address' => [
+                    'city' => 'New York',
+                    'zip' => '10001'
+                ]
+            ]
+        ]);
+
+        $this->assertEquals($expected, $serializer->serialize($input));
+    }
+
+    public function testSerializeWithObject()
+    {
+        $serializer = new JsonSerializer();
+        $input = (object)[
+            'name' => 'John Doe',
+            'age' => 30
+        ];
+        $expected = json_encode([
+            'name' => 'John Doe',
+            'age' => 30
+        ]);
+
+        $this->assertEquals($expected, $serializer->serialize($input));
+    }
+
+    public function testSerializeWithKeysTransformation()
+    {
+        $serializer = new JsonSerializer();
+        $input = [
+            'three_ds' => 'enabled',
+            'if_match' => 'etag_value'
+        ];
+        $expected = json_encode([
+            '3ds' => 'enabled',
+            'if-match' => 'etag_value'
+        ]);
+
+        $this->assertEquals($expected, $serializer->serialize($input));
+    }
+
+    public function testSerializeWithIterableObject()
+    {
+        $serializer = new JsonSerializer();
+
+        $iterableObject = new IterableObject([
+            'item1' => 'value1',
+            'item2' => 'value2'
+        ]);
+
+        $input = [
+            'iterable' => $iterableObject
+        ];
+        $expected = json_encode([
+            'iterable' => [
+                'item1' => 'value1',
+                'item2' => 'value2'
+            ]
+        ]);
+
+        $this->assertEquals($expected, $serializer->serialize($input));
+    }
+}