diff --git a/ExecutableFinder.php b/ExecutableFinder.php
index 5914b4cd..2fe558be 100644
--- a/ExecutableFinder.php
+++ b/ExecutableFinder.php
@@ -43,10 +43,8 @@ public function addSuffix(string $suffix)
      * @param string      $name      The executable name (without the extension)
      * @param string|null $default   The default to return if no executable is found
      * @param array       $extraDirs Additional dirs to check into
-     *
-     * @return string|null
      */
-    public function find(string $name, string $default = null, array $extraDirs = [])
+    public function find(string $name, string $default = null, array $extraDirs = []): ?string
     {
         if (ini_get('open_basedir')) {
             $searchPath = array_merge(explode(\PATH_SEPARATOR, ini_get('open_basedir')), $extraDirs);
diff --git a/InputStream.php b/InputStream.php
index 240665f3..b8682bae 100644
--- a/InputStream.php
+++ b/InputStream.php
@@ -41,7 +41,7 @@ public function onEmpty(callable $onEmpty = null)
      * @param resource|string|int|float|bool|\Traversable|null $input The input to append as scalar,
      *                                                                stream resource or \Traversable
      */
-    public function write($input)
+    public function write(mixed $input)
     {
         if (null === $input) {
             return;
@@ -68,11 +68,7 @@ public function isClosed()
         return !$this->open;
     }
 
-    /**
-     * @return \Traversable<int, string>
-     */
-    #[\ReturnTypeWillChange]
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         $this->open = true;
 
diff --git a/PhpExecutableFinder.php b/PhpExecutableFinder.php
index ec24f911..1881adaf 100644
--- a/PhpExecutableFinder.php
+++ b/PhpExecutableFinder.php
@@ -28,10 +28,8 @@ public function __construct()
 
     /**
      * Finds The PHP executable.
-     *
-     * @return string|false
      */
-    public function find(bool $includeArgs = true)
+    public function find(bool $includeArgs = true): string|false
     {
         if ($php = getenv('PHP_BINARY')) {
             if (!is_executable($php)) {
@@ -84,10 +82,8 @@ public function find(bool $includeArgs = true)
 
     /**
      * Finds the PHP executable arguments.
-     *
-     * @return array
      */
-    public function findArguments()
+    public function findArguments(): array
     {
         $arguments = [];
         if ('phpdbg' === \PHP_SAPI) {
diff --git a/PhpProcess.php b/PhpProcess.php
index 2bc338e5..cb749f9b 100644
--- a/PhpProcess.php
+++ b/PhpProcess.php
@@ -53,7 +53,7 @@ public function __construct(string $script, string $cwd = null, array $env = nul
     /**
      * {@inheritdoc}
      */
-    public static function fromShellCommandline(string $command, string $cwd = null, array $env = null, $input = null, ?float $timeout = 60)
+    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));
     }
diff --git a/Pipes/AbstractPipes.php b/Pipes/AbstractPipes.php
index 2d145872..dca7ae7f 100644
--- a/Pipes/AbstractPipes.php
+++ b/Pipes/AbstractPipes.php
@@ -20,7 +20,7 @@
  */
 abstract class AbstractPipes implements PipesInterface
 {
-    public $pipes = [];
+    public array $pipes = [];
 
     private $inputBuffer = '';
     private $input;
@@ -30,7 +30,7 @@ abstract class AbstractPipes implements PipesInterface
     /**
      * @param resource|string|int|float|bool|\Iterator|null $input
      */
-    public function __construct($input)
+    public function __construct(mixed $input)
     {
         if (\is_resource($input) || $input instanceof \Iterator) {
             $this->input = $input;
diff --git a/Pipes/UnixPipes.php b/Pipes/UnixPipes.php
index 5a0e9d47..063aa6ad 100644
--- a/Pipes/UnixPipes.php
+++ b/Pipes/UnixPipes.php
@@ -26,7 +26,7 @@ class UnixPipes extends AbstractPipes
     private $ptyMode;
     private $haveReadSupport;
 
-    public function __construct(?bool $ttyMode, bool $ptyMode, $input, bool $haveReadSupport)
+    public function __construct(?bool $ttyMode, bool $ptyMode, mixed $input, bool $haveReadSupport)
     {
         $this->ttyMode = $ttyMode;
         $this->ptyMode = $ptyMode;
diff --git a/Pipes/WindowsPipes.php b/Pipes/WindowsPipes.php
index bca84f57..e68ed951 100644
--- a/Pipes/WindowsPipes.php
+++ b/Pipes/WindowsPipes.php
@@ -35,7 +35,7 @@ class WindowsPipes extends AbstractPipes
     ];
     private $haveReadSupport;
 
-    public function __construct($input, bool $haveReadSupport)
+    public function __construct(mixed $input, bool $haveReadSupport)
     {
         $this->haveReadSupport = $haveReadSupport;
 
diff --git a/Process.php b/Process.php
index 4d811a69..878d7f58 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, $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.');
@@ -185,11 +185,9 @@ public function __construct(array $command, string $cwd = null, array $env = nul
      * @param mixed          $input   The input as stream resource, scalar or \Traversable, or null for no input
      * @param int|float|null $timeout The timeout in seconds or null to disable
      *
-     * @return static
-     *
      * @throws LogicException When proc_open is not installed
      */
-    public static function fromShellCommandline(string $command, string $cwd = null, array $env = null, $input = null, ?float $timeout = 60)
+    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;
@@ -197,10 +195,7 @@ public static function fromShellCommandline(string $command, string $cwd = null,
         return $process;
     }
 
-    /**
-     * @return array
-     */
-    public function __sleep()
+    public function __sleep(): array
     {
         throw new \BadMethodCallException('Cannot serialize '.__CLASS__);
     }
@@ -266,7 +261,7 @@ public function run(callable $callback = null, array $env = []): int
      *
      * @final
      */
-    public function mustRun(callable $callback = null, array $env = []): self
+    public function mustRun(callable $callback = null, array $env = []): static
     {
         if (0 !== $this->run($callback, $env)) {
             throw new ProcessFailedException($this);
@@ -376,8 +371,6 @@ public function start(callable $callback = null, array $env = [])
      * @param callable|null $callback A PHP callback to run whenever there is some
      *                                output available on STDOUT or STDERR
      *
-     * @return static
-     *
      * @throws RuntimeException When process can't be launched
      * @throws RuntimeException When process is already running
      *
@@ -385,7 +378,7 @@ public function start(callable $callback = null, array $env = [])
      *
      * @final
      */
-    public function restart(callable $callback = null, array $env = []): self
+    public function restart(callable $callback = null, array $env = []): static
     {
         if ($this->isRunning()) {
             throw new RuntimeException('Process is already running.');
@@ -412,7 +405,7 @@ public function restart(callable $callback = null, array $env = []): self
      * @throws ProcessSignaledException When process stopped after receiving signal
      * @throws LogicException           When process is not yet started
      */
-    public function wait(callable $callback = null)
+    public function wait(callable $callback = null): int
     {
         $this->requireProcessIsStarted(__FUNCTION__);
 
@@ -495,7 +488,7 @@ public function waitUntil(callable $callback): bool
      *
      * @return int|null The process id if running, null otherwise
      */
-    public function getPid()
+    public function getPid(): ?int
     {
         return $this->isRunning() ? $this->processInformation['pid'] : null;
     }
@@ -511,7 +504,7 @@ public function getPid()
      * @throws RuntimeException In case --enable-sigchild is activated and the process can't be killed
      * @throws RuntimeException In case of failure
      */
-    public function signal(int $signal)
+    public function signal(int $signal): static
     {
         $this->doSignal($signal, true);
 
@@ -526,7 +519,7 @@ public function signal(int $signal)
      * @throws RuntimeException In case the process is already running
      * @throws LogicException   if an idle timeout is set
      */
-    public function disableOutput()
+    public function disableOutput(): static
     {
         if ($this->isRunning()) {
             throw new RuntimeException('Disabling output while the process is running is not possible.');
@@ -547,7 +540,7 @@ public function disableOutput()
      *
      * @throws RuntimeException In case the process is already running
      */
-    public function enableOutput()
+    public function enableOutput(): static
     {
         if ($this->isRunning()) {
             throw new RuntimeException('Enabling output while the process is running is not possible.');
@@ -560,10 +553,8 @@ public function enableOutput()
 
     /**
      * Returns true in case the output is disabled, false otherwise.
-     *
-     * @return bool
      */
-    public function isOutputDisabled()
+    public function isOutputDisabled(): bool
     {
         return $this->outputDisabled;
     }
@@ -571,12 +562,10 @@ public function isOutputDisabled()
     /**
      * Returns the current output of the process (STDOUT).
      *
-     * @return string
-     *
      * @throws LogicException in case the output has been disabled
      * @throws LogicException In case the process is not started
      */
-    public function getOutput()
+    public function getOutput(): string
     {
         $this->readPipesForOutput(__FUNCTION__);
 
@@ -593,12 +582,10 @@ public function getOutput()
      * In comparison with the getOutput method which always return the whole
      * output, this one returns the new output since the last call.
      *
-     * @return string
-     *
      * @throws LogicException in case the output has been disabled
      * @throws LogicException In case the process is not started
      */
-    public function getIncrementalOutput()
+    public function getIncrementalOutput(): string
     {
         $this->readPipesForOutput(__FUNCTION__);
 
@@ -619,11 +606,8 @@ public function getIncrementalOutput()
      *
      * @throws LogicException in case the output has been disabled
      * @throws LogicException In case the process is not started
-     *
-     * @return \Generator<string, string>
      */
-    #[\ReturnTypeWillChange]
-    public function getIterator(int $flags = 0)
+    public function getIterator(int $flags = 0): \Generator
     {
         $this->readPipesForOutput(__FUNCTION__, false);
 
@@ -675,7 +659,7 @@ public function getIterator(int $flags = 0)
      *
      * @return $this
      */
-    public function clearOutput()
+    public function clearOutput(): static
     {
         ftruncate($this->stdout, 0);
         fseek($this->stdout, 0);
@@ -687,12 +671,10 @@ public function clearOutput()
     /**
      * Returns the current error output of the process (STDERR).
      *
-     * @return string
-     *
      * @throws LogicException in case the output has been disabled
      * @throws LogicException In case the process is not started
      */
-    public function getErrorOutput()
+    public function getErrorOutput(): string
     {
         $this->readPipesForOutput(__FUNCTION__);
 
@@ -710,12 +692,10 @@ public function getErrorOutput()
      * whole error output, this one returns the new error output since the last
      * call.
      *
-     * @return string
-     *
      * @throws LogicException in case the output has been disabled
      * @throws LogicException In case the process is not started
      */
-    public function getIncrementalErrorOutput()
+    public function getIncrementalErrorOutput(): string
     {
         $this->readPipesForOutput(__FUNCTION__);
 
@@ -734,7 +714,7 @@ public function getIncrementalErrorOutput()
      *
      * @return $this
      */
-    public function clearErrorOutput()
+    public function clearErrorOutput(): static
     {
         ftruncate($this->stderr, 0);
         fseek($this->stderr, 0);
@@ -748,7 +728,7 @@ public function clearErrorOutput()
      *
      * @return int|null The exit status code, null if the Process is not terminated
      */
-    public function getExitCode()
+    public function getExitCode(): ?int
     {
         $this->updateStatus(false);
 
@@ -766,7 +746,7 @@ public function getExitCode()
      * @see http://tldp.org/LDP/abs/html/exitcodes.html
      * @see http://en.wikipedia.org/wiki/Unix_signal
      */
-    public function getExitCodeText()
+    public function getExitCodeText(): ?string
     {
         if (null === $exitcode = $this->getExitCode()) {
             return null;
@@ -777,10 +757,8 @@ public function getExitCodeText()
 
     /**
      * Checks if the process ended successfully.
-     *
-     * @return bool
      */
-    public function isSuccessful()
+    public function isSuccessful(): bool
     {
         return 0 === $this->getExitCode();
     }
@@ -790,11 +768,9 @@ public function isSuccessful()
      *
      * It always returns false on Windows.
      *
-     * @return bool
-     *
      * @throws LogicException In case the process is not terminated
      */
-    public function hasBeenSignaled()
+    public function hasBeenSignaled(): bool
     {
         $this->requireProcessIsTerminated(__FUNCTION__);
 
@@ -806,12 +782,10 @@ public function hasBeenSignaled()
      *
      * It is only meaningful if hasBeenSignaled() returns true.
      *
-     * @return int
-     *
      * @throws RuntimeException In case --enable-sigchild is activated
      * @throws LogicException   In case the process is not terminated
      */
-    public function getTermSignal()
+    public function getTermSignal(): int
     {
         $this->requireProcessIsTerminated(__FUNCTION__);
 
@@ -827,11 +801,9 @@ public function getTermSignal()
      *
      * It always returns false on Windows.
      *
-     * @return bool
-     *
      * @throws LogicException In case the process is not terminated
      */
-    public function hasBeenStopped()
+    public function hasBeenStopped(): bool
     {
         $this->requireProcessIsTerminated(__FUNCTION__);
 
@@ -843,11 +815,9 @@ public function hasBeenStopped()
      *
      * It is only meaningful if hasBeenStopped() returns true.
      *
-     * @return int
-     *
      * @throws LogicException In case the process is not terminated
      */
-    public function getStopSignal()
+    public function getStopSignal(): int
     {
         $this->requireProcessIsTerminated(__FUNCTION__);
 
@@ -856,10 +826,8 @@ public function getStopSignal()
 
     /**
      * Checks if the process is currently running.
-     *
-     * @return bool
      */
-    public function isRunning()
+    public function isRunning(): bool
     {
         if (self::STATUS_STARTED !== $this->status) {
             return false;
@@ -872,20 +840,16 @@ public function isRunning()
 
     /**
      * Checks if the process has been started with no regard to the current state.
-     *
-     * @return bool
      */
-    public function isStarted()
+    public function isStarted(): bool
     {
         return self::STATUS_READY != $this->status;
     }
 
     /**
      * Checks if the process is terminated.
-     *
-     * @return bool
      */
-    public function isTerminated()
+    public function isTerminated(): bool
     {
         $this->updateStatus(false);
 
@@ -896,10 +860,8 @@ public function isTerminated()
      * Gets the process status.
      *
      * The status is one of: ready, started, terminated.
-     *
-     * @return string
      */
-    public function getStatus()
+    public function getStatus(): string
     {
         $this->updateStatus(false);
 
@@ -914,7 +876,7 @@ public function getStatus()
      *
      * @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)
+    public function stop(float $timeout = 10, int $signal = null): ?int
     {
         $timeoutMicro = microtime(true) + $timeout;
         if ($this->isRunning()) {
@@ -973,8 +935,6 @@ public function addErrorOutput(string $line)
 
     /**
      * Gets the last output time in seconds.
-     *
-     * @return float|null
      */
     public function getLastOutputTime(): ?float
     {
@@ -983,30 +943,24 @@ public function getLastOutputTime(): ?float
 
     /**
      * Gets the command line to be executed.
-     *
-     * @return string
      */
-    public function getCommandLine()
+    public function getCommandLine(): string
     {
         return \is_array($this->commandline) ? implode(' ', array_map([$this, 'escapeArgument'], $this->commandline)) : $this->commandline;
     }
 
     /**
      * Gets the process timeout in seconds (max. runtime).
-     *
-     * @return float|null
      */
-    public function getTimeout()
+    public function getTimeout(): ?float
     {
         return $this->timeout;
     }
 
     /**
      * Gets the process idle timeout in seconds (max. time since last output).
-     *
-     * @return float|null
      */
-    public function getIdleTimeout()
+    public function getIdleTimeout(): ?float
     {
         return $this->idleTimeout;
     }
@@ -1020,7 +974,7 @@ public function getIdleTimeout()
      *
      * @throws InvalidArgumentException if the timeout is negative
      */
-    public function setTimeout(?float $timeout)
+    public function setTimeout(?float $timeout): static
     {
         $this->timeout = $this->validateTimeout($timeout);
 
@@ -1037,7 +991,7 @@ public function setTimeout(?float $timeout)
      * @throws LogicException           if the output is disabled
      * @throws InvalidArgumentException if the timeout is negative
      */
-    public function setIdleTimeout(?float $timeout)
+    public function setIdleTimeout(?float $timeout): static
     {
         if (null !== $timeout && $this->outputDisabled) {
             throw new LogicException('Idle timeout cannot be set while the output is disabled.');
@@ -1055,7 +1009,7 @@ public function setIdleTimeout(?float $timeout)
      *
      * @throws RuntimeException In case the TTY mode is not supported
      */
-    public function setTty(bool $tty)
+    public function setTty(bool $tty): static
     {
         if ('\\' === \DIRECTORY_SEPARATOR && $tty) {
             throw new RuntimeException('TTY mode is not supported on Windows platform.');
@@ -1072,10 +1026,8 @@ public function setTty(bool $tty)
 
     /**
      * Checks if the TTY mode is enabled.
-     *
-     * @return bool
      */
-    public function isTty()
+    public function isTty(): bool
     {
         return $this->tty;
     }
@@ -1085,7 +1037,7 @@ public function isTty()
      *
      * @return $this
      */
-    public function setPty(bool $bool)
+    public function setPty(bool $bool): static
     {
         $this->pty = $bool;
 
@@ -1094,20 +1046,16 @@ public function setPty(bool $bool)
 
     /**
      * Returns PTY state.
-     *
-     * @return bool
      */
-    public function isPty()
+    public function isPty(): bool
     {
         return $this->pty;
     }
 
     /**
      * Gets the working directory.
-     *
-     * @return string|null
      */
-    public function getWorkingDirectory()
+    public function getWorkingDirectory(): ?string
     {
         if (null === $this->cwd) {
             // getcwd() will return false if any one of the parent directories does not have
@@ -1123,7 +1071,7 @@ public function getWorkingDirectory()
      *
      * @return $this
      */
-    public function setWorkingDirectory(string $cwd)
+    public function setWorkingDirectory(string $cwd): static
     {
         $this->cwd = $cwd;
 
@@ -1132,10 +1080,8 @@ public function setWorkingDirectory(string $cwd)
 
     /**
      * Gets the environment variables.
-     *
-     * @return array
      */
-    public function getEnv()
+    public function getEnv(): array
     {
         return $this->env;
     }
@@ -1147,7 +1093,7 @@ public function getEnv()
      *
      * @return $this
      */
-    public function setEnv(array $env)
+    public function setEnv(array $env): static
     {
         $this->env = $env;
 
@@ -1175,7 +1121,7 @@ public function getInput()
      *
      * @throws LogicException In case the process is running
      */
-    public function setInput($input)
+    public function setInput(mixed $input): static
     {
         if ($this->isRunning()) {
             throw new LogicException('Input cannot be set while the process is running.');
@@ -1267,10 +1213,8 @@ public static function isTtySupported(): bool
 
     /**
      * Returns whether PTY is supported on the current operating system.
-     *
-     * @return bool
      */
-    public static function isPtySupported()
+    public static function isPtySupported(): bool
     {
         static $result;
 
@@ -1309,10 +1253,8 @@ private function getDescriptors(): array
      * the user callback (if present) with the received output.
      *
      * @param callable|null $callback The user defined PHP callback
-     *
-     * @return \Closure
      */
-    protected function buildCallback(callable $callback = null)
+    protected function buildCallback(callable $callback = null): \Closure
     {
         if ($this->outputDisabled) {
             return function ($type, $data) use ($callback): bool {
@@ -1360,10 +1302,8 @@ protected function updateStatus(bool $blocking)
 
     /**
      * Returns whether PHP has been compiled with the '--enable-sigchild' option or not.
-     *
-     * @return bool
      */
-    protected function isSigchildEnabled()
+    protected function isSigchildEnabled(): bool
     {
         if (null !== self::$sigchild) {
             return self::$sigchild;
@@ -1493,8 +1433,6 @@ private function resetProcessData()
      * @param int  $signal         A valid POSIX signal (see https://php.net/pcntl.constants)
      * @param bool $throwException Whether to throw exception in case signal failed
      *
-     * @return bool
-     *
      * @throws LogicException   In case the process is not running
      * @throws RuntimeException In case --enable-sigchild is activated and the process can't be killed
      * @throws RuntimeException In case of failure
diff --git a/ProcessUtils.php b/ProcessUtils.php
index 6cc7a610..6fedd34d 100644
--- a/ProcessUtils.php
+++ b/ProcessUtils.php
@@ -35,11 +35,9 @@ private function __construct()
      * @param string $caller The name of method call that validates the input
      * @param mixed  $input  The input to validate
      *
-     * @return mixed
-     *
      * @throws InvalidArgumentException In case the input is not valid
      */
-    public static function validateInput(string $caller, $input)
+    public static function validateInput(string $caller, mixed $input): mixed
     {
         if (null !== $input) {
             if (\is_resource($input)) {
diff --git a/composer.json b/composer.json
index 1669eba5..9f1aa8cf 100644
--- a/composer.json
+++ b/composer.json
@@ -16,8 +16,7 @@
         }
     ],
     "require": {
-        "php": ">=7.2.5",
-        "symfony/polyfill-php80": "^1.16"
+        "php": ">=8.0.2"
     },
     "autoload": {
         "psr-4": { "Symfony\\Component\\Process\\": "" },
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index c32f2510..13bd3f83 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
 <phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/5.2/phpunit.xsd"
+         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/9.3/phpunit.xsd"
          backupGlobals="false"
          colors="true"
          bootstrap="vendor/autoload.php"
@@ -18,13 +18,13 @@
         </testsuite>
     </testsuites>
 
-    <filter>
-        <whitelist>
+    <coverage>
+        <include>
             <directory>./</directory>
-            <exclude>
-                <directory>./Tests</directory>
-                <directory>./vendor</directory>
-            </exclude>
-        </whitelist>
-    </filter>
+        </include>
+        <exclude>
+            <directory>./Tests</directory>
+            <directory>./vendor</directory>
+        </exclude>
+    </coverage>
 </phpunit>