diff --git a/modules/openapi-generator/src/main/resources/php-nextgen/ObjectSerializer.mustache b/modules/openapi-generator/src/main/resources/php-nextgen/ObjectSerializer.mustache
index 3e51b750323b..0b5d2288ed8e 100644
--- a/modules/openapi-generator/src/main/resources/php-nextgen/ObjectSerializer.mustache
+++ b/modules/openapi-generator/src/main/resources/php-nextgen/ObjectSerializer.mustache
@@ -198,6 +198,31 @@ class ObjectSerializer
         };
     }
 
+    /**
+     * Convert objects into arrays, recursively.
+     *
+     * sanitizeForSerialization() returns an object per model, but query parameters
+     * are flattened from arrays.
+     *
+     * @param mixed $data
+     *
+     * @return mixed the data with every object converted to an array
+     */
+    private static function toArrayRecursive(mixed $data): mixed
+    {
+        if (is_array($data) || is_object($data)) {
+            $result = [];
+
+            foreach ($data as $key => $value) {
+                $result[$key] = self::toArrayRecursive($value);
+            }
+
+            return $result;
+        }
+
+        return $data;
+    }
+
     /**
      * Take query parameter properties and turn it into an array suitable for
      * native http_build_query or GuzzleHttp\Psr7\Query::build.
@@ -237,8 +262,19 @@ class ObjectSerializer
             return ["{$paramName}" => $value->format(self::$dateTimeFormat)];
         }
 
+        // A model is typed with its class name rather than "object", but serializes as one.
+        if ($value instanceof ModelInterface) {
+            $openApiType = 'object';
+        }
+
         $query = [];
-        $value = (in_array($openApiType, ['object', 'array'], true)) ? (array)$value : $value;
+        if ($openApiType === 'object' && is_object($value)) {
+            // Read the model's values through its getters; a plain (array) cast
+            // would expose only the protected $container holding them.
+            $value = self::toArrayRecursive(self::sanitizeForSerialization($value));
+        } elseif (in_array($openApiType, ['object', 'array'], true)) {
+            $value = (array) $value;
+        }
 
         // since \GuzzleHttp\Psr7\Query::build fails with nested arrays
         // need to flatten array first
diff --git a/modules/openapi-generator/src/main/resources/php-nextgen/phpunit.xml.mustache b/modules/openapi-generator/src/main/resources/php-nextgen/phpunit.xml.mustache
index b72fb7c6e77e..bbad79749af4 100644
--- a/modules/openapi-generator/src/main/resources/php-nextgen/phpunit.xml.mustache
+++ b/modules/openapi-generator/src/main/resources/php-nextgen/phpunit.xml.mustache
@@ -8,8 +8,7 @@
   </coverage>
   <testsuites>
     <testsuite name="tests">
-      <directory>{{apiTestPath}}</directory>
-      <directory>{{modelTestPath}}</directory>
+      <directory>./{{testBasePath}}</directory>
     </testsuite>
   </testsuites>
   <php>
diff --git a/samples/client/echo_api/php-nextgen-streaming/phpunit.xml.dist b/samples/client/echo_api/php-nextgen-streaming/phpunit.xml.dist
index 1135449a8d9b..fff90cbd49db 100644
--- a/samples/client/echo_api/php-nextgen-streaming/phpunit.xml.dist
+++ b/samples/client/echo_api/php-nextgen-streaming/phpunit.xml.dist
@@ -8,8 +8,7 @@
   </coverage>
   <testsuites>
     <testsuite name="tests">
-      <directory>./tests/Api</directory>
-      <directory>./tests/Model</directory>
+      <directory>./tests</directory>
     </testsuite>
   </testsuites>
   <php>
diff --git a/samples/client/echo_api/php-nextgen-streaming/src/ObjectSerializer.php b/samples/client/echo_api/php-nextgen-streaming/src/ObjectSerializer.php
index 2b4404907885..4aca1d803fb7 100644
--- a/samples/client/echo_api/php-nextgen-streaming/src/ObjectSerializer.php
+++ b/samples/client/echo_api/php-nextgen-streaming/src/ObjectSerializer.php
@@ -207,6 +207,31 @@ private static function isEmptyValue(mixed $value, string $openApiType): bool
         };
     }
 
+    /**
+     * Convert objects into arrays, recursively.
+     *
+     * sanitizeForSerialization() returns an object per model, but query parameters
+     * are flattened from arrays.
+     *
+     * @param mixed $data
+     *
+     * @return mixed the data with every object converted to an array
+     */
+    private static function toArrayRecursive(mixed $data): mixed
+    {
+        if (is_array($data) || is_object($data)) {
+            $result = [];
+
+            foreach ($data as $key => $value) {
+                $result[$key] = self::toArrayRecursive($value);
+            }
+
+            return $result;
+        }
+
+        return $data;
+    }
+
     /**
      * Take query parameter properties and turn it into an array suitable for
      * native http_build_query or GuzzleHttp\Psr7\Query::build.
@@ -246,8 +271,19 @@ public static function toQueryValue(
             return ["{$paramName}" => $value->format(self::$dateTimeFormat)];
         }
 
+        // A model is typed with its class name rather than "object", but serializes as one.
+        if ($value instanceof ModelInterface) {
+            $openApiType = 'object';
+        }
+
         $query = [];
-        $value = (in_array($openApiType, ['object', 'array'], true)) ? (array)$value : $value;
+        if ($openApiType === 'object' && is_object($value)) {
+            // Read the model's values through its getters; a plain (array) cast
+            // would expose only the protected $container holding them.
+            $value = self::toArrayRecursive(self::sanitizeForSerialization($value));
+        } elseif (in_array($openApiType, ['object', 'array'], true)) {
+            $value = (array) $value;
+        }
 
         // since \GuzzleHttp\Psr7\Query::build fails with nested arrays
         // need to flatten array first
diff --git a/samples/client/echo_api/php-nextgen/phpunit.xml.dist b/samples/client/echo_api/php-nextgen/phpunit.xml.dist
index 1135449a8d9b..fff90cbd49db 100644
--- a/samples/client/echo_api/php-nextgen/phpunit.xml.dist
+++ b/samples/client/echo_api/php-nextgen/phpunit.xml.dist
@@ -8,8 +8,7 @@
   </coverage>
   <testsuites>
     <testsuite name="tests">
-      <directory>./tests/Api</directory>
-      <directory>./tests/Model</directory>
+      <directory>./tests</directory>
     </testsuite>
   </testsuites>
   <php>
diff --git a/samples/client/echo_api/php-nextgen/src/ObjectSerializer.php b/samples/client/echo_api/php-nextgen/src/ObjectSerializer.php
index 2b4404907885..4aca1d803fb7 100644
--- a/samples/client/echo_api/php-nextgen/src/ObjectSerializer.php
+++ b/samples/client/echo_api/php-nextgen/src/ObjectSerializer.php
@@ -207,6 +207,31 @@ private static function isEmptyValue(mixed $value, string $openApiType): bool
         };
     }
 
+    /**
+     * Convert objects into arrays, recursively.
+     *
+     * sanitizeForSerialization() returns an object per model, but query parameters
+     * are flattened from arrays.
+     *
+     * @param mixed $data
+     *
+     * @return mixed the data with every object converted to an array
+     */
+    private static function toArrayRecursive(mixed $data): mixed
+    {
+        if (is_array($data) || is_object($data)) {
+            $result = [];
+
+            foreach ($data as $key => $value) {
+                $result[$key] = self::toArrayRecursive($value);
+            }
+
+            return $result;
+        }
+
+        return $data;
+    }
+
     /**
      * Take query parameter properties and turn it into an array suitable for
      * native http_build_query or GuzzleHttp\Psr7\Query::build.
@@ -246,8 +271,19 @@ public static function toQueryValue(
             return ["{$paramName}" => $value->format(self::$dateTimeFormat)];
         }
 
+        // A model is typed with its class name rather than "object", but serializes as one.
+        if ($value instanceof ModelInterface) {
+            $openApiType = 'object';
+        }
+
         $query = [];
-        $value = (in_array($openApiType, ['object', 'array'], true)) ? (array)$value : $value;
+        if ($openApiType === 'object' && is_object($value)) {
+            // Read the model's values through its getters; a plain (array) cast
+            // would expose only the protected $container holding them.
+            $value = self::toArrayRecursive(self::sanitizeForSerialization($value));
+        } elseif (in_array($openApiType, ['object', 'array'], true)) {
+            $value = (array) $value;
+        }
 
         // since \GuzzleHttp\Psr7\Query::build fails with nested arrays
         // need to flatten array first
diff --git a/samples/client/petstore/php-nextgen/OpenAPIClient-php/phpunit.xml.dist b/samples/client/petstore/php-nextgen/OpenAPIClient-php/phpunit.xml.dist
index 1135449a8d9b..fff90cbd49db 100644
--- a/samples/client/petstore/php-nextgen/OpenAPIClient-php/phpunit.xml.dist
+++ b/samples/client/petstore/php-nextgen/OpenAPIClient-php/phpunit.xml.dist
@@ -8,8 +8,7 @@
   </coverage>
   <testsuites>
     <testsuite name="tests">
-      <directory>./tests/Api</directory>
-      <directory>./tests/Model</directory>
+      <directory>./tests</directory>
     </testsuite>
   </testsuites>
   <php>
diff --git a/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/ObjectSerializer.php b/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/ObjectSerializer.php
index fe0227e2f3c3..d6140c08d586 100644
--- a/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/ObjectSerializer.php
+++ b/samples/client/petstore/php-nextgen/OpenAPIClient-php/src/ObjectSerializer.php
@@ -206,6 +206,31 @@ private static function isEmptyValue(mixed $value, string $openApiType): bool
         };
     }
 
+    /**
+     * Convert objects into arrays, recursively.
+     *
+     * sanitizeForSerialization() returns an object per model, but query parameters
+     * are flattened from arrays.
+     *
+     * @param mixed $data
+     *
+     * @return mixed the data with every object converted to an array
+     */
+    private static function toArrayRecursive(mixed $data): mixed
+    {
+        if (is_array($data) || is_object($data)) {
+            $result = [];
+
+            foreach ($data as $key => $value) {
+                $result[$key] = self::toArrayRecursive($value);
+            }
+
+            return $result;
+        }
+
+        return $data;
+    }
+
     /**
      * Take query parameter properties and turn it into an array suitable for
      * native http_build_query or GuzzleHttp\Psr7\Query::build.
@@ -245,8 +270,19 @@ public static function toQueryValue(
             return ["{$paramName}" => $value->format(self::$dateTimeFormat)];
         }
 
+        // A model is typed with its class name rather than "object", but serializes as one.
+        if ($value instanceof ModelInterface) {
+            $openApiType = 'object';
+        }
+
         $query = [];
-        $value = (in_array($openApiType, ['object', 'array'], true)) ? (array)$value : $value;
+        if ($openApiType === 'object' && is_object($value)) {
+            // Read the model's values through its getters; a plain (array) cast
+            // would expose only the protected $container holding them.
+            $value = self::toArrayRecursive(self::sanitizeForSerialization($value));
+        } elseif (in_array($openApiType, ['object', 'array'], true)) {
+            $value = (array) $value;
+        }
 
         // since \GuzzleHttp\Psr7\Query::build fails with nested arrays
         // need to flatten array first
diff --git a/samples/client/petstore/php-nextgen/OpenAPIClient-php/tests/ObjectSerializerTest.php b/samples/client/petstore/php-nextgen/OpenAPIClient-php/tests/ObjectSerializerTest.php
new file mode 100644
index 000000000000..4000b29226ef
--- /dev/null
+++ b/samples/client/petstore/php-nextgen/OpenAPIClient-php/tests/ObjectSerializerTest.php
@@ -0,0 +1,248 @@
+<?php
+
+namespace OpenAPI\Client;
+
+use OpenAPI\Client\Model\ArrayTest;
+use OpenAPI\Client\Model\Category;
+use OpenAPI\Client\Model\Pet;
+use OpenAPI\Client\Model\Tag;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * class ObjectSerializerTest
+ *
+ * @package OpenAPI\Client
+ */
+class ObjectSerializerTest extends TestCase
+{
+    /**
+     * @covers ObjectSerializer::toQueryValue
+     * @dataProvider provideQueryParams
+     */
+    public function testToQueryValue(
+        mixed $data,
+        string $paramName,
+        string $openApiType,
+        string $style,
+        bool $explode,
+        bool $required,
+        mixed $expected
+    ): void {
+        $value = ObjectSerializer::toQueryValue($data, $paramName, $openApiType, $style, $explode, $required);
+        $query = ObjectSerializer::buildQuery($value);
+
+        $this->assertEquals($expected, $query);
+    }
+
+    /**
+     * Query params provider
+     *
+     * Values that are not model instances, whose serialization is unchanged.
+     *
+     * @return array[]
+     */
+    public static function provideQueryParams(): array
+    {
+        $statuses = ['available', 'pending', 'sold'];
+        $filter = ['name' => 'Rex', 'status' => 'available'];
+
+        $stdClass = new \stdClass();
+        $stdClass->name = 'Rex';
+        $stdClass->category = ['name' => 'Dogs'];
+
+        return [
+            // style form
+            // status=available&status=pending&status=sold
+            'form array, explode on, required true' => [
+                $statuses, 'status', 'array', 'form', true, true, 'status=available&status=pending&status=sold',
+            ],
+            // status=available,pending,sold
+            'form array, explode off, required true' => [
+                $statuses, 'status', 'array', 'form', false, true, 'status=available%2Cpending%2Csold',
+            ],
+            // name=Rex&status=available
+            'form object, explode on, required true' => [
+                $filter, 'filter', 'object', 'form', true, true, 'name=Rex&status=available',
+            ],
+            // filter=name,Rex,status,available
+            'form object, explode off, required true' => [
+                $filter, 'filter', 'object', 'form', false, true, 'filter=name%2CRex%2Cstatus%2Cavailable',
+            ],
+            // status=available
+            'form string, explode on, required true' => [
+                'available', 'status', 'string', 'form', true, true, 'status=available',
+            ],
+            // quantity=0
+            'form 0 integer, explode on, required false' => [
+                0, 'quantity', 'integer', 'form', true, false, 'quantity=0',
+            ],
+
+            // DEEP OBJECT
+            // status[0]=available&status[1]=pending&status[2]=sold
+            'deepObject array, explode on, required true' => [
+                $statuses, 'status', 'array', 'deepObject', true, true,
+                'status%5B0%5D=available&status%5B1%5D=pending&status%5B2%5D=sold',
+            ],
+            // filter[name]=Rex&filter[status]=available
+            'deepObject object, explode on, required true' => [
+                $filter, 'filter', 'object', 'deepObject', true, true,
+                'filter%5Bname%5D=Rex&filter%5Bstatus%5D=available',
+            ],
+            // filter[name]=Rex&filter[category][name]=Dogs
+            'deepObject stdClass, explode on, required true' => [
+                $stdClass, 'filter', 'object', 'deepObject', true, true,
+                'filter%5Bname%5D=Rex&filter%5Bcategory%5D%5Bname%5D=Dogs',
+            ],
+        ];
+    }
+
+    /**
+     * An object-typed query parameter given as a model must serialize to the model's
+     * values, read through its getters and keyed by its attributeMap - note photoUrls.
+     *
+     * @see https://github.com/OpenAPITools/openapi-generator/issues/11222
+     * @covers ObjectSerializer::toQueryValue
+     */
+    public function testToQueryValueWithModelInstance(): void
+    {
+        // photo_urls is required and non-nullable, so it must be set for its getter to return.
+        $pet = new Pet([
+            'id' => 1,
+            'name' => 'Rex',
+            'photo_urls' => ['a.png', 'b.png'],
+            'status' => 'available',
+        ]);
+
+        $query = ObjectSerializer::toQueryValue($pet, 'filter', 'object', 'deepObject', true, false);
+
+        $this->assertEquals(
+            'filter[id]=1&filter[name]=Rex&filter[photoUrls][0]=a.png'
+                . '&filter[photoUrls][1]=b.png&filter[status]=available',
+            urldecode(ObjectSerializer::buildQuery($query))
+        );
+    }
+
+    /**
+     * A composed (allOf/oneOf/anyOf) parameter is typed with a model name rather than
+     * "object", so models are recognised by their interface instead of that type.
+     *
+     * @covers ObjectSerializer::toQueryValue
+     * @dataProvider provideModelTypedStyles
+     */
+    public function testToQueryValueWithModelInstanceTypedByItsModelName(
+        string $style,
+        string $expected
+    ): void {
+        $pet = new Pet(['id' => 1, 'name' => 'Rex', 'photo_urls' => [], 'status' => 'available']);
+
+        $query = ObjectSerializer::toQueryValue($pet, 'filter', Pet::class, $style, true, false);
+
+        $this->assertSame($expected, urldecode(ObjectSerializer::buildQuery($query)));
+    }
+
+    /**
+     * Styles provider for a parameter typed with a model name
+     *
+     * @return array[]
+     */
+    public static function provideModelTypedStyles(): array
+    {
+        return [
+            'deepObject, explode on' => [
+                'deepObject', 'filter[id]=1&filter[name]=Rex&filter[status]=available',
+            ],
+            'form, explode on' => [
+                'form', 'id=1&name=Rex&status=available',
+            ],
+        ];
+    }
+
+    /**
+     * A model holding no values flattens to nothing, so the parameter is dropped instead
+     * of being sent as an empty container key.
+     *
+     * A deepObject has no key of its own to fall back on, so a required parameter is
+     * dropped just the same - hence the identical result for both.
+     *
+     * @covers ObjectSerializer::toQueryValue
+     */
+    public function testToQueryValueWithAnEmptyModelInstance(): void
+    {
+        $this->assertSame(
+            [],
+            ObjectSerializer::toQueryValue(new Tag(), 'filter', Tag::class, 'deepObject', true, false)
+        );
+        $this->assertSame(
+            [],
+            ObjectSerializer::toQueryValue(new Tag(), 'filter', Tag::class, 'deepObject', true, true)
+        );
+    }
+
+    /**
+     * Objects nested in a plain object are converted as well: a shallow (array) cast would
+     * leave them in place, and building the query would then fail to stringify them.
+     *
+     * @covers ObjectSerializer::toQueryValue
+     */
+    public function testToQueryValueWithAPlainObjectHoldingNestedObjects(): void
+    {
+        $filter = new \stdClass();
+        $filter->name = 'Rex';
+        $filter->tag = new Tag(['id' => 2, 'name' => 'stray']);
+        $filter->bornAt = new \DateTime('2024-01-02T03:04:05Z');
+
+        $query = ObjectSerializer::toQueryValue($filter, 'filter', 'object', 'deepObject', true, false);
+
+        $this->assertSame(
+            'filter[name]=Rex&filter[tag][id]=2&filter[tag][name]=stray'
+                . '&filter[bornAt]=2024-01-02T03:04:05+00:00',
+            urldecode(ObjectSerializer::buildQuery($query))
+        );
+    }
+
+    /**
+     * Array properties keep their indexes, at every depth.
+     *
+     * @covers ObjectSerializer::toQueryValue
+     */
+    public function testToQueryValueWithArrayPropertiesOfAModelInstance(): void
+    {
+        $model = new ArrayTest([
+            'array_of_string' => ['a', 'b'],
+            'array_array_of_integer' => [[1, 2]],
+        ]);
+
+        $query = ObjectSerializer::toQueryValue($model, 'filter', 'object', 'deepObject', true, false);
+
+        $this->assertEquals(
+            'filter[array_of_string][0]=a&filter[array_of_string][1]=b'
+                . '&filter[array_array_of_integer][0][0]=1&filter[array_array_of_integer][0][1]=2',
+            urldecode(ObjectSerializer::buildQuery($query))
+        );
+    }
+
+    /**
+     * Nested models must be flattened too, so the whole object graph is converted to
+     * arrays first: a shallow cast would leave them as objects and fail to stringify.
+     *
+     * @covers ObjectSerializer::toQueryValue
+     */
+    public function testToQueryValueWithNestedModelInstances(): void
+    {
+        $pet = new Pet([
+            'id' => 1,
+            'name' => 'Rex',
+            'photo_urls' => [],
+            'category' => new Category(['id' => 7, 'name' => 'Dogs']),
+            'tags' => [new Tag(['id' => 2, 'name' => 'cute'])],
+        ]);
+
+        $query = ObjectSerializer::toQueryValue($pet, 'filter', 'object', 'deepObject', true, false);
+
+        $this->assertEquals(
+            'filter[id]=1&filter[category][id]=7&filter[category][name]=Dogs'
+                . '&filter[name]=Rex&filter[tags][0][id]=2&filter[tags][0][name]=cute',
+            urldecode(ObjectSerializer::buildQuery($query))
+        );
+    }
+}