diff --git a/.gitattributes b/.gitattributes
index 7c0fa03..7950037 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -3,6 +3,8 @@
 .gitattributes   export-ignore
 .gitignore       export-ignore
 .scrutinizer.yml export-ignore
+.styleci.yml     export-ignore
+.php_cs.yml      export-ignore
 .travis.yml      export-ignore
 CONDUCT.md       export-ignore
 CONTRIBUTING.md  export-ignore
diff --git a/tests/BaseUnitTestCase.php b/tests/BaseUnitTestCase.php
index 1557431..3a3c9e3 100644
--- a/tests/BaseUnitTestCase.php
+++ b/tests/BaseUnitTestCase.php
@@ -4,7 +4,6 @@
 
 namespace Http\Client\Curl\Tests;
 
-use Http\Client\Curl\PromiseCore;
 use Http\Discovery\MessageFactoryDiscovery;
 use PHPUnit\Framework\TestCase;
 use Psr\Http\Message\RequestInterface;
@@ -37,11 +36,11 @@ protected function tearDown()
      * Create new request.
      *
      * @param string $method
-     * @param mixed  $uri
+     * @param mixed $uri
      *
      * @return RequestInterface
      */
-    protected function createRequest($method, $uri)
+    protected function createRequest(string $method, $uri): RequestInterface
     {
         return MessageFactoryDiscovery::find()->createRequest($method, $uri);
     }
@@ -51,18 +50,8 @@ protected function createRequest($method, $uri)
      *
      * @return ResponseInterface
      */
-    protected function createResponse()
+    protected function createResponse(): ResponseInterface
     {
         return MessageFactoryDiscovery::find()->createResponse();
     }
-
-    /**
-     * Create PromiseCore mock.
-     *
-     * @return PromiseCore|\PHPUnit_Framework_MockObject_MockObject
-     */
-    protected function createPromiseCore()
-    {
-        return $this->getMockBuilder(PromiseCore::class)->disableOriginalConstructor()->getMock();
-    }
 }
diff --git a/tests/ClientTest.php b/tests/ClientTest.php
index 85ae402..13c6dcd 100644
--- a/tests/ClientTest.php
+++ b/tests/ClientTest.php
@@ -25,7 +25,7 @@ class ClientTest extends TestCase
      */
     public function testExpectHeader()
     {
-        $client = $this->getMockBuilder(Client::class)->disableOriginalConstructor()->getMock();
+        $client = $this->createMock(Client::class);
 
         $createHeaders = new \ReflectionMethod(Client::class, 'createHeaders');
         $createHeaders->setAccessible(true);
@@ -44,7 +44,7 @@ public function testExpectHeader()
      */
     public function testWithNullPostFields()
     {
-        $client = $this->getMockBuilder(Client::class)->disableOriginalConstructor()->getMock();
+        $client = $this->createMock(Client::class);
 
         $createHeaders = new \ReflectionMethod(Client::class, 'createHeaders');
         $createHeaders->setAccessible(true);
@@ -59,7 +59,7 @@ public function testWithNullPostFields()
 
     public function testRewindStream()
     {
-        $client = $this->getMockBuilder(Client::class)->disableOriginalConstructor()->getMock();
+        $client = $this->createMock(Client::class);
 
         $bodyOptions = new \ReflectionMethod(Client::class, 'addRequestBodyOptions');
         $bodyOptions->setAccessible(true);
@@ -74,7 +74,7 @@ public function testRewindStream()
 
     public function testRewindLargeStream()
     {
-        $client = $this->getMockBuilder(Client::class)->disableOriginalConstructor()->getMock();
+        $client = $this->createMock(Client::class);
 
         $bodyOptions = new \ReflectionMethod(Client::class, 'addRequestBodyOptions');
         $bodyOptions->setAccessible(true);
diff --git a/tests/CurlPromiseTest.php b/tests/CurlPromiseTest.php
index 5c4ddf8..389262e 100644
--- a/tests/CurlPromiseTest.php
+++ b/tests/CurlPromiseTest.php
@@ -6,6 +6,7 @@
 
 use Http\Client\Curl\CurlPromise;
 use Http\Client\Curl\MultiRunner;
+use Http\Client\Curl\PromiseCore;
 use Http\Client\Exception\TransferException;
 use Http\Promise\Promise;
 
@@ -21,10 +22,9 @@ class CurlPromiseTest extends BaseUnitTestCase
      */
     public function testCoreCalls()
     {
-        $core = $this->createPromiseCore();
-        $runner = $this->getMockBuilder(MultiRunner::class)->disableOriginalConstructor()
-            ->setMethods(['wait'])->getMock();
-        /** @var MultiRunner|\PHPUnit_Framework_MockObject_MockObject $runner */
+        $core = $this->createMock(PromiseCore::class);
+        $runner = $this->createMock(MultiRunner::class);
+
         $promise = new CurlPromise($core, $runner);
 
         $onFulfill = function () {
@@ -42,10 +42,9 @@ public function testCoreCalls()
 
     public function testCoreCallWaitFulfilled()
     {
-        $core = $this->createPromiseCore();
-        $runner = $this->getMockBuilder(MultiRunner::class)->disableOriginalConstructor()
-            ->setMethods(['wait'])->getMock();
-        /** @var MultiRunner|\PHPUnit_Framework_MockObject_MockObject $runner */
+        $core = $this->createMock(PromiseCore::class);
+        $runner = $this->createMock(MultiRunner::class);
+
         $promise = new CurlPromise($core, $runner);
 
         $runner->expects(static::once())->method('wait')->with($core);
@@ -56,9 +55,9 @@ public function testCoreCallWaitFulfilled()
 
     public function testCoreCallWaitRejected()
     {
-        $core = $this->createPromiseCore();
-        $runner = $this->getMockBuilder(MultiRunner::class)->disableOriginalConstructor()->getMock();
-        /** @var MultiRunner|\PHPUnit_Framework_MockObject_MockObject $runner */
+        $core = $this->createMock(PromiseCore::class);
+        $runner = $this->createMock(MultiRunner::class);
+
         $promise = new CurlPromise($core, $runner);
 
         $runner->expects(static::once())->method('wait')->with($core);
diff --git a/tests/HttpAsyncClientTestCase.php b/tests/HttpAsyncClientTestCase.php
index 8a6f7f0..0b3f839 100644
--- a/tests/HttpAsyncClientTestCase.php
+++ b/tests/HttpAsyncClientTestCase.php
@@ -17,11 +17,8 @@ abstract class HttpAsyncClientTestCase extends HttpAsyncClientTest
      */
     public function testAsyncSendRequest($method, $uri, array $headers, $body)
     {
-        if (defined('HHVM_VERSION')) {
-            static::markTestSkipped('This test can not run under HHVM');
-        }
         if (null !== $body && in_array($method, ['GET', 'HEAD', 'TRACE'], true)) {
-            static::markTestSkipped('cURL can not send body using '.$method);
+            static::markTestSkipped('cURL can not send body using ' . $method);
         }
         parent::testAsyncSendRequest(
             $method,
@@ -41,9 +38,6 @@ public function testSendAsyncRequestWithOutcome(
         array $headers,
         $body
     ) {
-        if (defined('HHVM_VERSION')) {
-            static::markTestSkipped('This test can not run under HHVM');
-        }
         if (null !== $body) {
             static::markTestSkipped('cURL can not send body using GET');
         }
@@ -54,12 +48,4 @@ public function testSendAsyncRequestWithOutcome(
             $body
         );
     }
-
-    public function testSuccessiveCallMustUseResponseInterface()
-    {
-        if (defined('HHVM_VERSION')) {
-            static::markTestSkipped('This test can not run under HHVM');
-        }
-        parent::testSuccessiveCallMustUseResponseInterface();
-    }
 }
diff --git a/tests/HttpClientTestCase.php b/tests/HttpClientTestCase.php
index cea9e90..25ee940 100644
--- a/tests/HttpClientTestCase.php
+++ b/tests/HttpClientTestCase.php
@@ -26,9 +26,6 @@ abstract class HttpClientTestCase extends HttpClientTest
      */
     public function testSendRequest($method, $uri, array $headers, $body)
     {
-        if (defined('HHVM_VERSION')) {
-            static::markTestSkipped('This test can not run under HHVM');
-        }
         if (null !== $body && in_array($method, ['GET', 'HEAD', 'TRACE'], true)) {
             static::markTestSkipped('cURL can not send body using '.$method);
         }
@@ -50,9 +47,6 @@ public function testSendRequestWithOutcome(
         array $headers,
         $body
     ) {
-        if (defined('HHVM_VERSION')) {
-            static::markTestSkipped('This test can not run under HHVM');
-        }
         if (null !== $body) {
             static::markTestSkipped('cURL can not send body using GET');
         }