diff --git a/.gitignore b/.gitignore
index 3e4ac79..9e9e092 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,3 +4,4 @@ vendor/
 .DS_Store
 composer.lock
 phpunit.xml
+.phpunit.result.cache
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index 1122286..0000000
--- a/.travis.yml
+++ /dev/null
@@ -1,25 +0,0 @@
-language: php
-
-php:
-  - 5.3
-  - 5.4
-  - 5.5
-  - 5.6
-  - 7.0
-  - hhvm
-
-matrix:
-  allow_failures:
-    - php: 7.0
-  include:
-    - php: 5.3
-      env:
-        - COMPOSER_FLAGS="--prefer-stable --prefer-lowest"
-        - PHPUNIT_FLAGS="-c phpunit.xml.ci"
-        - COVERAGE=true
-
-install:
-  - travis_retry composer self-update
-  - travis_retry composer update ${COMPOSER_FLAGS} --prefer-source --no-interaction
-
-script: vendor/bin/phpunit ${PHPUNIT_FLAGS}
diff --git a/README.md b/README.md
index 37432fa..4d03575 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@ without affecting the memory footprint.
 Via Composer
 
 ``` bash
-$ composer require th3n3rd/cartesian-product
+$ composer require "th3n3rd/cartesian-product"
 ```
 
 ## Usage
@@ -28,18 +28,18 @@ use Nerd\CartesianProduct\CartesianProduct;
 $cartesianProduct = new CartesianProduct();
 
 $cartesianProduct
-    ->appendSet(array('a', 'b', 'c'))
-    ->appendSet(array('d', 'e'))
-    ->appendSet(array('f', 'g', 'h'))
-    ->appendSet(array('i', 'j'))
-    ->appendSet(array('k', 'l'))
-    ->appendSet(array('m', 'n'))
-    ->appendSet(array('o'))
-    ->appendSet(array('p'))
-    ->appendSet(array('q', 'r', 's', 't'))
-    ->appendSet(array('u', 'v', 'w'))
-    ->appendSet(array('x', 'y'))
-    ->appendSet(array('z'))
+    ->appendSet(['a', 'b', 'c'])
+    ->appendSet(['d', 'e'])
+    ->appendSet(['f', 'g', 'h'])
+    ->appendSet(['i', 'j'])
+    ->appendSet(['k', 'l'])
+    ->appendSet(['m', 'n'])
+    ->appendSet(['o'])
+    ->appendSet(['p'])
+    ->appendSet(['q', 'r', 's', 't'])
+    ->appendSet(['u', 'v', 'w'])
+    ->appendSet(['x', 'y'])
+    ->appendSet(['z'])
 ;
 
 foreach ($cartesianProduct as $index => $product) {
@@ -58,7 +58,7 @@ foreach ($result as $index => $product) {
 ## Testing
 
 ``` bash
-$ phpunit
+$ vendor/bin/phpunit
 ```
 
 ## License
diff --git a/composer.json b/composer.json
index 31ed908..4ff6801 100644
--- a/composer.json
+++ b/composer.json
@@ -10,11 +10,11 @@
         }
     ],
     "require": {
-        "php": ">=5.3.3"
+        "php": ">=8.0"
     },
     "require-dev": {
-        "devster/ubench": "~1.1",
-        "phpunit/phpunit": "~4.4"
+        "devster/ubench": "^2.1",
+        "phpunit/phpunit": "^9.6.17"
     },
     "autoload": {
         "psr-4": {
diff --git a/examples/iterator.php b/examples/iterator.php
index e8f7a43..a1b2644 100644
--- a/examples/iterator.php
+++ b/examples/iterator.php
@@ -17,18 +17,18 @@
 $cartesianProduct = new CartesianProduct();
 
 $cartesianProduct
-    ->appendSet(array('a', 'b', 'c'))
-    ->appendSet(array('d', 'e'))
-    ->appendSet(array('f', 'g', 'h'))
-    ->appendSet(array('i', 'j'))
-    ->appendSet(array('k', 'l'))
-    ->appendSet(array('m', 'n'))
-    ->appendSet(array('o'))
-    ->appendSet(array('p'))
-    ->appendSet(array('q', 'r', 's', 't'))
-    ->appendSet(array('u', 'v', 'w'))
-    ->appendSet(array('x', 'y'))
-    ->appendSet(array('z'))
+    ->appendSet(['a', 'b', 'c'])
+    ->appendSet(['d', 'e'])
+    ->appendSet(['f', 'g', 'h'])
+    ->appendSet(['i', 'j'])
+    ->appendSet(['k', 'l'])
+    ->appendSet(['m', 'n'])
+    ->appendSet(['o'])
+    ->appendSet(['p'])
+    ->appendSet(['q', 'r', 's', 't'])
+    ->appendSet(['u', 'v', 'w'])
+    ->appendSet(['x', 'y'])
+    ->appendSet(['z'])
 ;
 
 $bench->start();
@@ -38,4 +38,4 @@
 $bench->end();
 
 printf("Time elapsed: %s\n", $bench->getTime());
-printf("Memory footprint: %s\n", $bench->getMemoryPeak());
\ No newline at end of file
+printf("Memory footprint: %s\n", $bench->getMemoryPeak());
diff --git a/examples/memory-usage-comparison.php b/examples/memory-usage-comparison.php
index 3bdcbb8..7bffe8b 100644
--- a/examples/memory-usage-comparison.php
+++ b/examples/memory-usage-comparison.php
@@ -17,18 +17,18 @@
 $cartesianProduct = new CartesianProduct();
 
 $cartesianProduct
-    ->appendSet(array('a', 'b', 'c'))
-    ->appendSet(array('d', 'e'))
-    ->appendSet(array('f', 'g', 'h'))
-    ->appendSet(array('i', 'j'))
-    ->appendSet(array('k', 'l'))
-    ->appendSet(array('m', 'n'))
-    ->appendSet(array('o'))
-    ->appendSet(array('p'))
-    ->appendSet(array('q', 'r', 's', 't'))
-    ->appendSet(array('u', 'v', 'w'))
-    ->appendSet(array('x', 'y'))
-    ->appendSet(array('z'))
+    ->appendSet(['a', 'b', 'c'])
+    ->appendSet(['d', 'e'])
+    ->appendSet(['f', 'g', 'h'])
+    ->appendSet(['i', 'j'])
+    ->appendSet(['k', 'l'])
+    ->appendSet(['m', 'n'])
+    ->appendSet(['o'])
+    ->appendSet(['p'])
+    ->appendSet(['q', 'r', 's', 't'])
+    ->appendSet(['u', 'v', 'w'])
+    ->appendSet(['x', 'y'])
+    ->appendSet(['z'])
 ;
 
 $bench->start();
@@ -52,4 +52,4 @@
 printf("Memory Usage Comparison: \n");
 printf("Whole -> %s\n", $wholeUseCase);
 printf("Iterator -> %s\n", $iteratorUseCase);
-printf("Ratio: 1:%s (whole vs iterator) \n", ceil($iteratorUseCaseBytes / $wholeUseCaseBytes * 100));
\ No newline at end of file
+printf("Ratio: 1:%s (whole vs iterator) \n", ceil($iteratorUseCaseBytes / $wholeUseCaseBytes * 100));
diff --git a/examples/whole-result.php b/examples/whole-result.php
index d70b028..d069ebb 100644
--- a/examples/whole-result.php
+++ b/examples/whole-result.php
@@ -17,18 +17,18 @@
 $cartesianProduct = new CartesianProduct();
 
 $cartesianProduct
-    ->appendSet(array('a', 'b', 'c'))
-    ->appendSet(array('d', 'e'))
-    ->appendSet(array('f', 'g', 'h'))
-    ->appendSet(array('i', 'j'))
-    ->appendSet(array('k', 'l'))
-    ->appendSet(array('m', 'n'))
-    ->appendSet(array('o'))
-    ->appendSet(array('p'))
-    ->appendSet(array('q', 'r', 's', 't'))
-    ->appendSet(array('u', 'v', 'w'))
-    ->appendSet(array('x', 'y'))
-    ->appendSet(array('z'))
+    ->appendSet(['a', 'b', 'c'])
+    ->appendSet(['d', 'e'])
+    ->appendSet(['f', 'g', 'h'])
+    ->appendSet(['i', 'j'])
+    ->appendSet(['k', 'l'])
+    ->appendSet(['m', 'n'])
+    ->appendSet(['o'])
+    ->appendSet(['p'])
+    ->appendSet(['q', 'r', 's', 't'])
+    ->appendSet(['u', 'v', 'w'])
+    ->appendSet(['x', 'y'])
+    ->appendSet(['z'])
 ;
 
 $bench->start();
@@ -39,4 +39,4 @@
 $bench->end();
 
 printf("Time elapsed: %s\n", $bench->getTime());
-printf("Memory footprint: %s\n", $bench->getMemoryPeak());
\ No newline at end of file
+printf("Memory footprint: %s\n", $bench->getMemoryPeak());
diff --git a/phpunit.xml.ci b/phpunit.xml.ci
deleted file mode 100644
index a42ac43..0000000
--- a/phpunit.xml.ci
+++ /dev/null
@@ -1,17 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<phpunit colors="true" bootstrap="vendor/autoload.php">
-    <testsuites>
-        <testsuite name="Cartesian Product Test Suite">
-            <directory>tests/</directory>
-        </testsuite>
-    </testsuites>
-    <filter>
-        <whitelist>
-            <directory suffix=".php">src/</directory>
-        </whitelist>
-    </filter>
-    <logging>
-        <log type="coverage-clover" target="build/coverage.xml" charset="UTF-8"/>
-        <log type="coverage-text" target="php://stdout" showUncoveredFiles="true"/>
-    </logging>
-</phpunit>
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 63da1e0..7cc8f42 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,13 +1,19 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit colors="true" bootstrap="vendor/autoload.php">
-    <testsuites>
-        <testsuite name="Cartesian Product Test Suite">
-            <directory>tests/</directory>
-        </testsuite>
-    </testsuites>
-    <filter>
-        <whitelist>
-            <directory suffix=".php">src/</directory>
-        </whitelist>
-    </filter>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         colors="true"
+         bootstrap="vendor/autoload.php"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+>
+  <coverage>
+    <include>
+      <directory suffix=".php">src/</directory>
+    </include>
+  </coverage>
+
+  <testsuites>
+    <testsuite name="Cartesian Product Test Suite">
+      <directory>tests/</directory>
+    </testsuite>
+  </testsuites>
+
 </phpunit>
diff --git a/src/CartesianProduct.php b/src/CartesianProduct.php
index 05d8725..3d0f584 100644
--- a/src/CartesianProduct.php
+++ b/src/CartesianProduct.php
@@ -16,25 +16,12 @@
  */
 class CartesianProduct implements \Iterator
 {
-    /**
-     * @var array
-     */
-    private $sets = array();
+    private array $sets = [];
+    private \Iterator $referenceSet;
 
-    /**
-     * @var \Iterator
-     */
-    private $referenceSet;
+    private int $cursor = 0;
 
-    /**
-     * @var integer
-     */
-    private $cursor = 0;
-
-    /**
-     * @param array $sets
-     */
-    public function __construct(array $sets = array())
+    public function __construct(array $sets = [])
     {
         foreach ($sets as $set) {
             $this->addSet($set);
@@ -46,11 +33,9 @@ public function __construct(array $sets = array())
     /**
      * Adds a set.
      *
-     * @param array|Traversable $set
-     *
      * @throws \InvalidArgumentException
      */
-    private function addSet($set)
+    private function addSet(iterable $set): void
     {
         if (is_array($set)) {
             $set = new \ArrayIterator($set);
@@ -66,13 +51,11 @@ private function addSet($set)
     /**
      * Appends the given set.
      *
-     * @param array|Traversable $set
-     *
      * @return $this
      *
      * @throws \InvalidArgumentException
      */
-    public function appendSet($set)
+    public function appendSet(iterable $set): static
     {
         $this->addSet($set);
         $this->computeReferenceSet();
@@ -83,7 +66,7 @@ public function appendSet($set)
     /**
      * Computes the reference set used for iterate over the product.
      */
-    private function computeReferenceSet()
+    private function computeReferenceSet(): void
     {
         if (empty($this->sets)) {
             return;
@@ -100,7 +83,7 @@ private function computeReferenceSet()
     /**
      * {@inheritdoc}
      */
-    public function current()
+    public function current(): mixed
     {
         $current = $this->referenceSet->current();
 
@@ -110,7 +93,7 @@ public function current()
     /**
      * {@inheritdoc}
      */
-    public function next()
+    public function next(): void
     {
         $this->cursor++;
         $this->referenceSet->next();
@@ -119,7 +102,7 @@ public function next()
     /**
      * {@inheritdoc}
      */
-    public function key()
+    public function key(): mixed
     {
         return $this->cursor;
     }
@@ -127,7 +110,7 @@ public function key()
     /**
      * {@inheritdoc}
      */
-    public function valid()
+    public function valid(): bool
     {
         return $this->referenceSet->valid();
     }
@@ -135,7 +118,7 @@ public function valid()
     /**
      * {@inheritdoc}
      */
-    public function rewind()
+    public function rewind(): void
     {
         $this->cursor = 0;
         $this->referenceSet->rewind();
@@ -149,9 +132,9 @@ public function rewind()
      * The recommended way to use the Cartesian Product is through its iterator interface
      * which is memory efficient.
      */
-    public function compute()
+    public function compute(): array
     {
-        $product = array();
+        $product = [];
 
         $this->referenceSet->rewind();
 
diff --git a/src/Set.php b/src/Set.php
index 74c09e5..90a85ce 100644
--- a/src/Set.php
+++ b/src/Set.php
@@ -16,19 +16,8 @@
  */
 class Set extends \IteratorIterator
 {
-    /**
-     * @var \Iterator
-     */
-    private $neighbour;
-
-    /**
-     * @param \Iterator $set
-     * @param \Iterator $neighbour
-     */
-    public function __construct(\Iterator $set, \Iterator $neighbour)
+    public function __construct(\Iterator $set, private \Iterator $neighbour)
     {
-        $this->neighbour = $neighbour;
-
         parent::__construct($set);
         parent::rewind();
     }
@@ -36,12 +25,12 @@ public function __construct(\Iterator $set, \Iterator $neighbour)
     /**
      * {@inheritdoc}
      */
-    public function current()
+    public function current(): mixed
     {
         $neighbourCurrent = $this->neighbour->current();
         $current = parent::current();
 
-        if (!$this->neighbour instanceof Set) {
+        if (!$this->neighbour instanceof self) {
             $neighbourCurrent = array($neighbourCurrent);
         }
 
@@ -53,7 +42,7 @@ public function current()
     /**
      * {@inheritdoc}
      */
-    public function next()
+    public function next(): void
     {
         $this->neighbour->next();
 
@@ -66,7 +55,7 @@ public function next()
     /**
      * {@inheritdoc}
      */
-    public function rewind()
+    public function rewind(): void
     {
         $this->neighbour->rewind();
         parent::rewind();
diff --git a/tests/CartesianProductTest.php b/tests/CartesianProductTest.php
index ba03e61..309a44e 100644
--- a/tests/CartesianProductTest.php
+++ b/tests/CartesianProductTest.php
@@ -11,32 +11,28 @@
 
 namespace Nerd\CartesianProduct;
 
+use PHPUnit\Framework\TestCase;
+
 /**
  * @author Marco Garofalo <marcogarofalo.personal@gmail.com>
  */
-class CartesianProductTest extends \PHPUnit_Framework_TestCase
+class CartesianProductTest extends TestCase
 {
-    /**
-     * @var CartesianProduct
-     */
-    private $cartesianProduct;
+    private CartesianProduct $cartesianProduct;
 
-    /**
-     * @var array
-     */
-    private static $sets = array(
-        array('a', 'b'),
-        array('c', 'd')
-    );
+    private static array $sets = [
+        ['a', 'b'],
+        ['c', 'd']
+    ];
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->cartesianProduct = new CartesianProduct(self::$sets);
     }
 
     public function testShouldBeAbleToHandleASingleSet()
     {
-        $singleSet = array(array('a', 'b'));
+        $singleSet = [['a', 'b']];
         $cartesianProduct = new CartesianProduct($singleSet);
 
         $this->assertTrue(is_array($cartesianProduct->current()));
@@ -46,15 +42,15 @@ public function testShouldBeAbleToHandleASingleSet()
 
     public function testShouldComputeTheCartesianProductIterativelyAndAsWholeCorrectly()
     {
-        $expectedProduct = array(
-            array('a', 'c'),
-            array('a', 'd'),
-            array('b', 'c'),
-            array('b', 'd'),
-        );
+        $expectedProduct = [
+            ['a', 'c'],
+            ['a', 'd'],
+            ['b', 'c'],
+            ['b', 'd'],
+        ];
 
         // using the iterator interface
-        $actualProductIteratively = array();
+        $actualProductIteratively = [];
         foreach ($this->cartesianProduct as $product) {
             $actualProductIteratively[] = $product;
         }
diff --git a/tests/SetTest.php b/tests/SetTest.php
index 93b9e0a..6ddda2b 100644
--- a/tests/SetTest.php
+++ b/tests/SetTest.php
@@ -11,15 +11,14 @@
 
 namespace Nerd\CartesianProduct;
 
+use PHPUnit\Framework\TestCase;
+
 /**
  * @author Marco Garofalo <marcogarofalo.personal@gmail.com>
  */
-class SetTest extends \PHPUnit_Framework_TestCase
+class SetTest extends TestCase
 {
-    /**
-     * @var array
-     */
-    private static $values = array('a', 'b');
+    private static array $values = array('a', 'b');
 
     public function testShouldComputeTheCurrentElementForBothArrayAndNonArrayValues()
     {