diff --git a/composer.json b/composer.json
index 5ea7fe4..0dc4761 100644
--- a/composer.json
+++ b/composer.json
@@ -13,7 +13,7 @@
         "sabre/uri"    : "^2.3"
     },
     "require-dev" : {
-        "friendsofphp/php-cs-fixer": "^3.38",
+        "friendsofphp/php-cs-fixer": "^3.54",
         "phpstan/phpstan": "^1.10",
         "phpunit/phpunit" : "^9.6"
     },
@@ -60,5 +60,10 @@
             "composer cs-fixer",
             "composer phpunit"
         ]
+    },
+    "config": {
+        "allow-plugins": {
+            "phpstan/extension-installer": true
+        }
     }
 }
diff --git a/examples/stringify.php b/examples/stringify.php
index 96f7da8..8ceefe2 100644
--- a/examples/stringify.php
+++ b/examples/stringify.php
@@ -30,7 +30,7 @@
 $request->setHeaders([
     'Host' => 'example.org',
     'Content-Type' => 'application/json',
-    ]);
+]);
 
 $request->setBody(json_encode(['foo' => 'bar']));
 
@@ -41,7 +41,7 @@
 $response->setHeaders([
     'Content-Type' => 'text/plain',
     'Connection' => 'close',
-    ]);
+]);
 
 $response->setBody('ABORT! ABORT!');
 
diff --git a/lib/Client.php b/lib/Client.php
index b73bf73..f94d933 100644
--- a/lib/Client.php
+++ b/lib/Client.php
@@ -177,7 +177,7 @@ public function send(RequestInterface $request): ResponseInterface
      * After calling sendAsync, you must therefore occasionally call the poll()
      * method, or wait().
      */
-    public function sendAsync(RequestInterface $request, callable $success = null, callable $error = null): void
+    public function sendAsync(RequestInterface $request, ?callable $success = null, ?callable $error = null): void
     {
         $this->emit('beforeRequest', [$request]);
         $this->sendAsyncInternal($request, $success, $error);
diff --git a/lib/Response.php b/lib/Response.php
index 19ac87b..934d724 100644
--- a/lib/Response.php
+++ b/lib/Response.php
@@ -101,7 +101,7 @@ class Response extends Message implements ResponseInterface
      * @param array<string, mixed>|null     $headers
      * @param resource|string|callable|null $body
      */
-    public function __construct($status = 500, array $headers = null, $body = null)
+    public function __construct($status = 500, ?array $headers = null, $body = null)
     {
         if (null !== $status) {
             $this->setStatus($status);
diff --git a/tests/HTTP/ClientTest.php b/tests/HTTP/ClientTest.php
index 4c1d5dc..61ed840 100644
--- a/tests/HTTP/ClientTest.php
+++ b/tests/HTTP/ClientTest.php
@@ -14,15 +14,15 @@ public function testCreateCurlSettingsArrayGET(): void
         $request = new Request('GET', 'http://example.org/', ['X-Foo' => 'bar']);
 
         $settings = [
-                CURLOPT_RETURNTRANSFER => true,
-                CURLOPT_HEADER => true,
-                CURLOPT_POSTREDIR => 0,
-                CURLOPT_HTTPHEADER => ['X-Foo: bar'],
-                CURLOPT_NOBODY => false,
-                CURLOPT_URL => 'http://example.org/',
-                CURLOPT_CUSTOMREQUEST => 'GET',
-                CURLOPT_USERAGENT => 'sabre-http/'.Version::VERSION.' (http://sabre.io/)',
-            ];
+            CURLOPT_RETURNTRANSFER => true,
+            CURLOPT_HEADER => true,
+            CURLOPT_POSTREDIR => 0,
+            CURLOPT_HTTPHEADER => ['X-Foo: bar'],
+            CURLOPT_NOBODY => false,
+            CURLOPT_URL => 'http://example.org/',
+            CURLOPT_CUSTOMREQUEST => 'GET',
+            CURLOPT_USERAGENT => 'sabre-http/'.Version::VERSION.' (http://sabre.io/)',
+        ];
 
         // FIXME: CURLOPT_PROTOCOLS and CURLOPT_REDIR_PROTOCOLS are currently unsupported by HHVM
         // at least if this unit test fails in the future we know it is :)
@@ -67,14 +67,14 @@ public function testCreateCurlSettingsArrayHEAD(): void
         $request = new Request('HEAD', 'http://example.org/', ['X-Foo' => 'bar']);
 
         $settings = [
-                CURLOPT_RETURNTRANSFER => true,
-                CURLOPT_HEADER => true,
-                CURLOPT_NOBODY => true,
-                CURLOPT_CUSTOMREQUEST => 'HEAD',
-                CURLOPT_HTTPHEADER => ['X-Foo: bar'],
-                CURLOPT_URL => 'http://example.org/',
-                CURLOPT_USERAGENT => 'sabre-http/'.Version::VERSION.' (http://sabre.io/)',
-            ];
+            CURLOPT_RETURNTRANSFER => true,
+            CURLOPT_HEADER => true,
+            CURLOPT_NOBODY => true,
+            CURLOPT_CUSTOMREQUEST => 'HEAD',
+            CURLOPT_HTTPHEADER => ['X-Foo: bar'],
+            CURLOPT_URL => 'http://example.org/',
+            CURLOPT_USERAGENT => 'sabre-http/'.Version::VERSION.' (http://sabre.io/)',
+        ];
 
         // FIXME: CURLOPT_PROTOCOLS and CURLOPT_REDIR_PROTOCOLS are currently unsupported by HHVM
         // at least if this unit test fails in the future we know it is :)
@@ -100,14 +100,14 @@ public function testCreateCurlSettingsArrayGETAfterHEAD(): void
         $request = new Request('GET', 'http://example.org/', ['X-Foo' => 'bar']);
 
         $settings = [
-                CURLOPT_CUSTOMREQUEST => 'GET',
-                CURLOPT_RETURNTRANSFER => true,
-                CURLOPT_HEADER => true,
-                CURLOPT_HTTPHEADER => ['X-Foo: bar'],
-                CURLOPT_NOBODY => false,
-                CURLOPT_URL => 'http://example.org/',
-                CURLOPT_USERAGENT => 'sabre-http/'.Version::VERSION.' (http://sabre.io/)',
-            ];
+            CURLOPT_CUSTOMREQUEST => 'GET',
+            CURLOPT_RETURNTRANSFER => true,
+            CURLOPT_HEADER => true,
+            CURLOPT_HTTPHEADER => ['X-Foo: bar'],
+            CURLOPT_NOBODY => false,
+            CURLOPT_URL => 'http://example.org/',
+            CURLOPT_USERAGENT => 'sabre-http/'.Version::VERSION.' (http://sabre.io/)',
+        ];
 
         // FIXME: CURLOPT_PROTOCOLS and CURLOPT_REDIR_PROTOCOLS are currently unsupported by HHVM
         // at least if this unit test fails in the future we know it is :)
@@ -129,17 +129,17 @@ public function testCreateCurlSettingsArrayPUTStream(): void
         $request = new Request('PUT', 'http://example.org/', ['X-Foo' => 'bar'], $h);
 
         $settings = [
-                CURLOPT_RETURNTRANSFER => true,
-                CURLOPT_HEADER => true,
-                CURLOPT_PUT => true,
-                CURLOPT_INFILE => $h,
-                CURLOPT_INFILESIZE => strlen($fileContent),
-                CURLOPT_NOBODY => false,
-                CURLOPT_CUSTOMREQUEST => 'PUT',
-                CURLOPT_HTTPHEADER => ['X-Foo: bar'],
-                CURLOPT_URL => 'http://example.org/',
-                CURLOPT_USERAGENT => 'sabre-http/'.Version::VERSION.' (http://sabre.io/)',
-            ];
+            CURLOPT_RETURNTRANSFER => true,
+            CURLOPT_HEADER => true,
+            CURLOPT_PUT => true,
+            CURLOPT_INFILE => $h,
+            CURLOPT_INFILESIZE => strlen($fileContent),
+            CURLOPT_NOBODY => false,
+            CURLOPT_CUSTOMREQUEST => 'PUT',
+            CURLOPT_HTTPHEADER => ['X-Foo: bar'],
+            CURLOPT_URL => 'http://example.org/',
+            CURLOPT_USERAGENT => 'sabre-http/'.Version::VERSION.' (http://sabre.io/)',
+        ];
 
         // FIXME: CURLOPT_PROTOCOLS and CURLOPT_REDIR_PROTOCOLS are currently unsupported by HHVM
         // at least if this unit test fails in the future we know it is :)
@@ -157,15 +157,15 @@ public function testCreateCurlSettingsArrayPUTString(): void
         $request = new Request('PUT', 'http://example.org/', ['X-Foo' => 'bar'], 'boo');
 
         $settings = [
-                CURLOPT_RETURNTRANSFER => true,
-                CURLOPT_HEADER => true,
-                CURLOPT_NOBODY => false,
-                CURLOPT_POSTFIELDS => 'boo',
-                CURLOPT_CUSTOMREQUEST => 'PUT',
-                CURLOPT_HTTPHEADER => ['X-Foo: bar'],
-                CURLOPT_URL => 'http://example.org/',
-                CURLOPT_USERAGENT => 'sabre-http/'.Version::VERSION.' (http://sabre.io/)',
-            ];
+            CURLOPT_RETURNTRANSFER => true,
+            CURLOPT_HEADER => true,
+            CURLOPT_NOBODY => false,
+            CURLOPT_POSTFIELDS => 'boo',
+            CURLOPT_CUSTOMREQUEST => 'PUT',
+            CURLOPT_HTTPHEADER => ['X-Foo: bar'],
+            CURLOPT_URL => 'http://example.org/',
+            CURLOPT_USERAGENT => 'sabre-http/'.Version::VERSION.' (http://sabre.io/)',
+        ];
 
         // FIXME: CURLOPT_PROTOCOLS and CURLOPT_REDIR_PROTOCOLS are currently unsupported by HHVM
         // at least if this unit test fails in the future we know it is :)
diff --git a/tests/HTTP/MessageDecoratorTest.php b/tests/HTTP/MessageDecoratorTest.php
index b5e1aa2..bc3e73e 100644
--- a/tests/HTTP/MessageDecoratorTest.php
+++ b/tests/HTTP/MessageDecoratorTest.php
@@ -30,7 +30,7 @@ public function testHeaders(): void
     {
         $this->outer->setHeaders([
             'a' => 'b',
-            ]);
+        ]);
 
         $this->assertEquals(['a' => ['b']], $this->inner->getHeaders());
         $this->assertEquals(['a' => ['b']], $this->outer->getHeaders());
@@ -44,7 +44,7 @@ public function testHeaders(): void
 
         $this->outer->addHeaders([
             'e' => 'f',
-            ]);
+        ]);
 
         $this->assertEquals(['a' => ['b'], 'c' => ['d'], 'e' => ['f']], $this->inner->getHeaders());
         $this->assertEquals(['a' => ['b'], 'c' => ['d'], 'e' => ['f']], $this->outer->getHeaders());