diff --git a/ExecutableFinder.php b/ExecutableFinder.php
index 2f0f68c1..a4747707 100644
--- a/ExecutableFinder.php
+++ b/ExecutableFinder.php
@@ -44,7 +44,7 @@ public function addSuffix(string $suffix): void
      * @param string|null $default   The default to return if no executable is found
      * @param array       $extraDirs Additional dirs to check into
      */
-    public function find(string $name, string $default = null, array $extraDirs = []): ?string
+    public function find(string $name, ?string $default = null, array $extraDirs = []): ?string
     {
         $dirs = array_merge(
             explode(\PATH_SEPARATOR, getenv('PATH') ?: getenv('Path')),
diff --git a/InputStream.php b/InputStream.php
index d0517de2..cd91029e 100644
--- a/InputStream.php
+++ b/InputStream.php
@@ -29,7 +29,7 @@ class InputStream implements \IteratorAggregate
     /**
      * Sets a callback that is called when the write buffer becomes empty.
      */
-    public function onEmpty(callable $onEmpty = null): void
+    public function onEmpty(?callable $onEmpty = null): void
     {
         $this->onEmpty = null !== $onEmpty ? $onEmpty(...) : null;
     }
diff --git a/PhpProcess.php b/PhpProcess.php
index 0d31e26f..01d88954 100644
--- a/PhpProcess.php
+++ b/PhpProcess.php
@@ -32,7 +32,7 @@ class PhpProcess extends Process
      * @param int         $timeout The timeout in seconds
      * @param array|null  $php     Path to the PHP binary to use with any additional arguments
      */
-    public function __construct(string $script, string $cwd = null, array $env = null, int $timeout = 60, array $php = null)
+    public function __construct(string $script, ?string $cwd = null, ?array $env = null, int $timeout = 60, ?array $php = null)
     {
         if (null === $php) {
             $executableFinder = new PhpExecutableFinder();
@@ -50,12 +50,12 @@ public function __construct(string $script, string $cwd = null, array $env = nul
         parent::__construct($php, $cwd, $env, $script, $timeout);
     }
 
-    public static function fromShellCommandline(string $command, string $cwd = null, array $env = null, mixed $input = null, ?float $timeout = 60): static
+    public static function fromShellCommandline(string $command, ?string $cwd = null, ?array $env = null, mixed $input = null, ?float $timeout = 60): static
     {
         throw new LogicException(sprintf('The "%s()" method cannot be called when using "%s".', __METHOD__, self::class));
     }
 
-    public function start(callable $callback = null, array $env = []): void
+    public function start(?callable $callback = null, array $env = []): void
     {
         if (null === $this->getCommandLine()) {
             throw new RuntimeException('Unable to find the PHP executable.');
diff --git a/PhpSubprocess.php b/PhpSubprocess.php
index 0720520f..a97f8b26 100644
--- a/PhpSubprocess.php
+++ b/PhpSubprocess.php
@@ -51,7 +51,7 @@ class PhpSubprocess extends Process
      * @param int         $timeout The timeout in seconds
      * @param array|null  $php     Path to the PHP binary to use with any additional arguments
      */
-    public function __construct(array $command, string $cwd = null, array $env = null, int $timeout = 60, array $php = null)
+    public function __construct(array $command, ?string $cwd = null, ?array $env = null, int $timeout = 60, ?array $php = null)
     {
         if (null === $php) {
             $executableFinder = new PhpExecutableFinder();
@@ -73,12 +73,12 @@ public function __construct(array $command, string $cwd = null, array $env = nul
         parent::__construct($command, $cwd, $env, null, $timeout);
     }
 
-    public static function fromShellCommandline(string $command, string $cwd = null, array $env = null, mixed $input = null, ?float $timeout = 60): static
+    public static function fromShellCommandline(string $command, ?string $cwd = null, ?array $env = null, mixed $input = null, ?float $timeout = 60): static
     {
         throw new LogicException(sprintf('The "%s()" method cannot be called when using "%s".', __METHOD__, self::class));
     }
 
-    public function start(callable $callback = null, array $env = []): void
+    public function start(?callable $callback = null, array $env = []): void
     {
         if (null === $this->getCommandLine()) {
             throw new RuntimeException('Unable to find the PHP executable.');
diff --git a/Process.php b/Process.php
index 6b73c31d..ed0507ec 100644
--- a/Process.php
+++ b/Process.php
@@ -140,7 +140,7 @@ class Process implements \IteratorAggregate
      *
      * @throws LogicException When proc_open is not installed
      */
-    public function __construct(array $command, string $cwd = null, array $env = null, mixed $input = null, ?float $timeout = 60)
+    public function __construct(array $command, ?string $cwd = null, ?array $env = null, mixed $input = null, ?float $timeout = 60)
     {
         if (!\function_exists('proc_open')) {
             throw new LogicException('The Process class relies on proc_open, which is not available on your PHP installation.');
@@ -186,7 +186,7 @@ public function __construct(array $command, string $cwd = null, array $env = nul
      *
      * @throws LogicException When proc_open is not installed
      */
-    public static function fromShellCommandline(string $command, string $cwd = null, array $env = null, mixed $input = null, ?float $timeout = 60): static
+    public static function fromShellCommandline(string $command, ?string $cwd = null, ?array $env = null, mixed $input = null, ?float $timeout = 60): static
     {
         $process = new static([], $cwd, $env, $input, $timeout);
         $process->commandline = $command;
@@ -241,7 +241,7 @@ public function __clone()
      *
      * @final
      */
-    public function run(callable $callback = null, array $env = []): int
+    public function run(?callable $callback = null, array $env = []): int
     {
         $this->start($callback, $env);
 
@@ -260,7 +260,7 @@ public function run(callable $callback = null, array $env = []): int
      *
      * @final
      */
-    public function mustRun(callable $callback = null, array $env = []): static
+    public function mustRun(?callable $callback = null, array $env = []): static
     {
         if (0 !== $this->run($callback, $env)) {
             throw new ProcessFailedException($this);
@@ -288,7 +288,7 @@ public function mustRun(callable $callback = null, array $env = []): static
      * @throws RuntimeException When process is already running
      * @throws LogicException   In case a callback is provided and output has been disabled
      */
-    public function start(callable $callback = null, array $env = []): void
+    public function start(?callable $callback = null, array $env = []): void
     {
         if ($this->isRunning()) {
             throw new RuntimeException('Process is already running.');
@@ -373,7 +373,7 @@ public function start(callable $callback = null, array $env = []): void
      *
      * @final
      */
-    public function restart(callable $callback = null, array $env = []): static
+    public function restart(?callable $callback = null, array $env = []): static
     {
         if ($this->isRunning()) {
             throw new RuntimeException('Process is already running.');
@@ -400,7 +400,7 @@ public function restart(callable $callback = null, array $env = []): static
      * @throws ProcessSignaledException When process stopped after receiving signal
      * @throws LogicException           When process is not yet started
      */
-    public function wait(callable $callback = null): int
+    public function wait(?callable $callback = null): int
     {
         $this->requireProcessIsStarted(__FUNCTION__);
 
@@ -873,7 +873,7 @@ public function getStatus(): string
      *
      * @return int|null The exit-code of the process or null if it's not running
      */
-    public function stop(float $timeout = 10, int $signal = null): ?int
+    public function stop(float $timeout = 10, ?int $signal = null): ?int
     {
         $timeoutMicro = microtime(true) + $timeout;
         if ($this->isRunning()) {
@@ -1247,7 +1247,7 @@ private function getDescriptors(bool $hasCallback): array
      *
      * @param callable|null $callback The user defined PHP callback
      */
-    protected function buildCallback(callable $callback = null): \Closure
+    protected function buildCallback(?callable $callback = null): \Closure
     {
         if ($this->outputDisabled) {
             return fn ($type, $data): bool => null !== $callback && $callback($type, $data);
diff --git a/Tests/ProcessTest.php b/Tests/ProcessTest.php
index 44fb54ee..cfea99cc 100644
--- a/Tests/ProcessTest.php
+++ b/Tests/ProcessTest.php
@@ -1566,7 +1566,7 @@ public function testNotTerminableInputPipe()
         $this->assertFalse($process->isRunning());
     }
 
-    private function getProcess(string|array $commandline, string $cwd = null, array $env = null, mixed $input = null, ?int $timeout = 60): Process
+    private function getProcess(string|array $commandline, ?string $cwd = null, ?array $env = null, mixed $input = null, ?int $timeout = 60): Process
     {
         if (\is_string($commandline)) {
             $process = Process::fromShellCommandline($commandline, $cwd, $env, $input, $timeout);
@@ -1579,7 +1579,7 @@ private function getProcess(string|array $commandline, string $cwd = null, array
         return self::$process = $process;
     }
 
-    private function getProcessForCode(string $code, string $cwd = null, array $env = null, $input = null, ?int $timeout = 60): Process
+    private function getProcessForCode(string $code, ?string $cwd = null, ?array $env = null, $input = null, ?int $timeout = 60): Process
     {
         return $this->getProcess([self::$phpBin, '-r', $code], $cwd, $env, $input, $timeout);
     }