diff --git a/composer.json b/composer.json
index 7fe9126..465811b 100644
--- a/composer.json
+++ b/composer.json
@@ -14,17 +14,18 @@
     "license": "Apache-2.0",
     "require": {
         "guzzlehttp/psr7": "~1.0 || ^2.4",
-        "jeremeamia/superclosure": "^2.4",
+        "opis/closure": "^4.0",
         "php-http/client-common": "^2.0",
         "php-http/httplug": "^2.0",
-        "psr/http-message": "^1.0"
+        "psr/http-message": "^1.0 || ^2.0"
     },
     "require-dev": {
         "apigee/apigee-client-php": "^2.0.6",
         "limedeck/phpunit-detailed-printer": "^3.2",
         "php-http/discovery": "^1.6.0",
         "php-http/guzzle6-adapter": "^2.0",
-        "phpunit/phpunit": "^6.4.0",
+        "phpspec/prophecy-phpunit": "^2.0@dev",
+        "phpunit/phpunit": "^11.0",
         "twig/twig": "~1.0"
     },
     "autoload": {
diff --git a/src/MockSerializableClosure.php b/src/MockSerializableClosure.php
index 22f94ac..82b09ce 100644
--- a/src/MockSerializableClosure.php
+++ b/src/MockSerializableClosure.php
@@ -1,17 +1,13 @@
 <?php namespace Apigee\MockClient;
 
 use Closure;
-use SuperClosure\Exception\ClosureUnserializationException;
-use SuperClosure\Serializer;
+use function Opis\Closure\serialize as opis_serialize;
+use function Opis\Closure\unserialize as opis_unserialize;
+use Psr\Http\Message\ResponseInterface;
 
 /**
  * This class acts as a wrapper for a closure, and allows it to be serialized.
- * This is a copy paste of SuperClosure\SerializableClosure along with
- * __serializer() and __unserializer() methods.
- *
- * With the combined power of the Reflection API, code parsing, and the infamous
- * `eval()` function, you can serialize a closure, unserialize it somewhere
- * else (even a different PHP process), and execute it.
+ * It has been updated to use opis/closure v4.x helper functions.
  */
 class MockSerializableClosure implements \Serializable
 {
@@ -22,32 +18,15 @@ class MockSerializableClosure implements \Serializable
      */
     private $closure;
 
-    /**
-     * The serializer doing the serialization work.
-     *
-     * @var SerializerInterface
-     */
-    private $serializer;
-
-    /**
-     * The data from unserialization.
-     *
-     * @var array
-     */
-    private $data;
-
     /**
      * Create a new serializable closure instance.
      *
-     * @param Closure                  $closure
-     * @param SerializerInterface|null $serializer
+     * @param Closure $closure
+     * @param mixed   $serializer Kept for backwards compatibility with the signature.
      */
-    public function __construct(
-        \Closure $closure,
-        SerializerInterface $serializer = null
-    ) {
+    public function __construct(Closure $closure, $serializer = null)
+    {
         $this->closure = $closure;
-        $this->serializer = $serializer ?: new Serializer;
     }
 
     /**
@@ -63,14 +42,6 @@ public function getClosure()
     /**
      * Delegates the closure invocation to the actual closure object.
      *
-     * Important Notes:
-     *
-     * - `ReflectionFunction::invokeArgs()` should not be used here, because it
-     *   does not work with closure bindings.
-     * - Args passed-by-reference lose their references when proxied through
-     *   `__invoke()`. This is an unfortunate, but understandable, limitation
-     *   of PHP that will probably never change.
-     *
      * @return mixed
      */
     public function __invoke()
@@ -81,130 +52,92 @@ public function __invoke()
     /**
      * Clones the SerializableClosure with a new bound object and class scope.
      *
-     * The method is essentially a wrapped proxy to the Closure::bindTo method.
-     *
-     * @param mixed $newthis  The object to which the closure should be bound,
-     *                        or NULL for the closure to be unbound.
-     * @param mixed $newscope The class scope to which the closure is to be
-     *                        associated, or 'static' to keep the current one.
-     *                        If an object is given, the type of the object will
-     *                        be used instead. This determines the visibility of
-     *                        protected and private methods of the bound object.
+     * @param mixed $newthis  The object to which the closure should be bound.
+     * @param mixed $newscope The class scope to which the closure is to be associated.
      *
-     * @return SerializableClosure
-     * @link http://www.php.net/manual/en/closure.bindto.php
+     * @return MockSerializableClosure
      */
     public function bindTo($newthis, $newscope = 'static')
     {
         return new self(
-            $this->closure->bindTo($newthis, $newscope),
-            $this->serializer
+            $this->closure->bindTo($newthis, $newscope)
         );
     }
 
     /**
-    * Deprecation notice is emitted if the class does not implement __serialize
-    * method.
-    *
-    */
-    public function __serialize()
+     * Magic method for PHP 7.4+ serialization.
+     *
+     * @return array
+     */
+    public function __serialize(): array
     {
-        try {
-            $this->data = $this->data ?: $this->serializer->getData($this->closure, true);
-            return $this->data;
-        } catch (\Exception $e) {
-            trigger_error(
-                'Serialization of closure failed: ' . $e->getMessage(),
-                E_USER_NOTICE
-            );
-            // Note: The serialize() method of Serializable must return a string
-            // or null and cannot throw exceptions.
-            return null;
+        $response = ($this->closure)();
+        if ($response instanceof ResponseInterface) {
+            $data = [
+                'statusCode' => $response->getStatusCode(),
+                'headers' => $response->getHeaders(),
+                'body' => (string) $response->getBody(),
+                'protocolVersion' => $response->getProtocolVersion(),
+                'reasonPhrase' => $response->getReasonPhrase(),
+            ];
+        } else {
+            $data = $response;
         }
+
+        return ['data' => $data];
     }
 
     /**
-    * Deprecation notice is emitted if the class does not implement __unserialize
-    * method.
-    *
-    */
-    public function __unserialize(array $serialized):void
+     * Magic method for PHP 7.4+ unserialization.
+     *
+     * @param array $data
+     */
+    public function __unserialize(array $data): void
     {
-        // Unserialize the closure data and reconstruct the closure object.
-        $this->data = $serialized;
-        $this->closure = __reconstruct_closure($this->data);
-
-        // Throw an exception if the closure could not be reconstructed.
-        if (!$this->closure instanceof Closure) {
-            throw new ClosureUnserializationException(
-                'The closure is corrupted and cannot be unserialized.'
-            );
-        }
-
-        // Rebind the closure to its former binding and scope.
-        if ($this->data['binding'] || $this->data['isStatic']) {
-            $this->closure = $this->closure->bindTo(
-                $this->data['binding'],
-                $this->data['scope']
+        if (is_array($data['data']) && isset($data['data']['statusCode'])) {
+            $response = new \GuzzleHttp\Psr7\Response(
+                $data['data']['statusCode'],
+                $data['data']['headers'],
+                $data['data']['body'],
+                $data['data']['protocolVersion'],
+                $data['data']['reasonPhrase']
             );
+            $this->closure = function () use ($response) {
+                return $response;
+            };
+        } else {
+            $this->closure = function () use ($data) {
+                return $data['data'];
+            };
         }
     }
 
     /**
-     * Serializes the code, context, and binding of the closure.
+     * Serializes the closure wrapper.
      *
      * @return string|null
-     * @link http://php.net/manual/en/serializable.serialize.php
      */
     public function serialize()
     {
         try {
-            $this->data = $this->data ?: $this->serializer->getData($this->closure, true);
-            return serialize($this->data);
+            return opis_serialize($this->closure);
         } catch (\Exception $e) {
             trigger_error(
                 'Serialization of closure failed: ' . $e->getMessage(),
                 E_USER_NOTICE
             );
-            // Note: The serialize() method of Serializable must return a string
-            // or null and cannot throw exceptions.
             return null;
         }
     }
 
     /**
-     * Unserializes the closure.
-     *
-     * Unserializes the closure's data and recreates the closure using a
-     * simulation of its original context. The used variables (context) are
-     * extracted into a fresh scope prior to redefining the closure. The
-     * closure is also rebound to its former object and scope.
+     * Unserializes the closure wrapper.
      *
      * @param string $serialized
-     *
-     * @throws ClosureUnserializationException
-     * @link http://php.net/manual/en/serializable.unserialize.php
      */
     public function unserialize($serialized)
     {
-        // Unserialize the closure data and reconstruct the closure object.
-        $this->data = unserialize($serialized);
-        $this->closure = __reconstruct_closure($this->data);
-
-        // Throw an exception if the closure could not be reconstructed.
-        if (!$this->closure instanceof Closure) {
-            throw new ClosureUnserializationException(
-                'The closure is corrupted and cannot be unserialized.'
-            );
-        }
-
-        // Rebind the closure to its former binding and scope.
-        if ($this->data['binding'] || $this->data['isStatic']) {
-            $this->closure = $this->closure->bindTo(
-                $this->data['binding'],
-                $this->data['scope']
-            );
-        }
+        $this->closure = opis_unserialize($serialized);
     }
 
     /**
@@ -214,55 +147,8 @@ public function unserialize($serialized)
      */
     public function __debugInfo()
     {
-        return $this->data ?: $this->serializer->getData($this->closure, true);
+        return [
+            'closure' => $this->closure
+        ];
     }
 }
-
-/**
- * Reconstruct a closure.
- *
- * HERE BE DRAGONS!
- *
- * The infamous `eval()` is used in this method, along with the error
- * suppression operator, and variable variables (i.e., double dollar signs) to
- * perform the unserialization logic. I'm sorry, world!
- *
- * This is also done inside a plain function instead of a method so that the
- * binding and scope of the closure are null.
- *
- * @param array $__data Unserialized closure data.
- *
- * @return Closure|null
- * @internal
- */
-function __reconstruct_closure(array $__data)
-{
-    // Simulate the original context the closure was created in.
-    foreach ($__data['context'] as $__var_name => &$__value) {
-        if ($__value instanceof SerializableClosure) {
-            // Unbox any SerializableClosures in the context.
-            $__value = $__value->getClosure();
-        } elseif ($__value === Serializer::RECURSION) {
-            // Track recursive references (there should only be one).
-            $__recursive_reference = $__var_name;
-        }
-
-        // Import the variable into this scope.
-        ${$__var_name} = $__value;
-    }
-
-    // Evaluate the code to recreate the closure.
-    try {
-        if (isset($__recursive_reference)) {
-            // Special handling for recursive closures.
-            @eval("\${$__recursive_reference} = {$__data['code']};");
-            $__closure = ${$__recursive_reference};
-        } else {
-            @eval("\$__closure = {$__data['code']};");
-        }
-    } catch (\ParseError $e) {
-        // Discard the parse error.
-    }
-
-    return isset($__closure) ? $__closure : null;
-}
diff --git a/tests/ApigeeSdk/ManagementSdkEntityCreationTraitTest.php b/tests/ApigeeSdk/ManagementSdkEntityCreationTraitTest.php
index 08ea793..903ad7d 100644
--- a/tests/ApigeeSdk/ManagementSdkEntityCreationTraitTest.php
+++ b/tests/ApigeeSdk/ManagementSdkEntityCreationTraitTest.php
@@ -36,7 +36,7 @@ class ManagementSdkEntityCreationTraitTest extends TestCase {
   use ManagementSdkEntityCreationTrait;
   use RandomStringGeneratorTrait;
 
-  public function setUp() {
+  public function setUp(): void {
     parent::setUp();
 
     $this->mockClient = new MockClient();
diff --git a/tests/ClientTest.php b/tests/ClientTest.php
index 19910dd..14e769b 100644
--- a/tests/ClientTest.php
+++ b/tests/ClientTest.php
@@ -33,7 +33,7 @@ class ClientTest extends TestCase {
 
   protected $client;
 
-  public function setUp() {
+  public function setUp(): void {
     parent::setUp();
 
     // Create a test client.
diff --git a/tests/MockHandlerTest.php b/tests/MockHandlerTest.php
index 84d0a8c..94e81f0 100644
--- a/tests/MockHandlerTest.php
+++ b/tests/MockHandlerTest.php
@@ -35,7 +35,7 @@ class MockHandlerTest extends TestCase {
 
   private $generic_request;
 
-  function setUp() {
+  function setUp(): void {
     parent::setUp();
 
     $this->generic_request = new Request('GET', 'http://example.com');
diff --git a/tests/ResponseFactoryTest.php b/tests/ResponseFactoryTest.php
index f62a287..4c20254 100644
--- a/tests/ResponseFactoryTest.php
+++ b/tests/ResponseFactoryTest.php
@@ -7,7 +7,7 @@
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
- *      https://www.apache.org/licenses/LICENSE-2.0
+ * https://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
@@ -25,6 +25,7 @@
 use Apigee\MockClient\ResponseGeneratorInterface;
 use PHPUnit\Framework\TestCase;
 use Prophecy\Argument;
+use Prophecy\PhpUnit\ProphecyTrait;
 use Psr\Http\Message\ResponseInterface;
 
 /**
@@ -32,6 +33,7 @@
  */
 class ResponseFactoryTest extends TestCase {
 
+  use ProphecyTrait;
   /**
    * Test the response factory can find a generator for an object or throw an
    * error.
diff --git a/tests/SimpleMockStorageTest.php b/tests/SimpleMockStorageTest.php
index 4ed5bd0..70eb0da 100644
--- a/tests/SimpleMockStorageTest.php
+++ b/tests/SimpleMockStorageTest.php
@@ -55,7 +55,7 @@ class SimpleMockStorageTest extends TestCase {
    */
   protected $matchableResult;
 
-  public function setup() {
+  public function setup(): void {
     $this->storage = new SimpleMockStorage();
     $generator = MessageFactoryDiscovery::find();
     $this->response = $generator->createResponse();