diff --git a/.travis.yml b/.travis.yml
index edf1451..2c21772 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,6 +4,9 @@ php:
   - 7.1
   - 7.2
   - 7.3
+  - 7.4
+  - 8.0
+  - 8.1
 branches:
   only:
     - master
@@ -16,4 +19,4 @@ script:
 after_script:
   - travis_retry vendor/bin/php-coveralls
   # or enable logging
-  - travis_retry vendor/bin/php-coveralls -v
\ No newline at end of file
+  - travis_retry vendor/bin/php-coveralls -v
diff --git a/composer.json b/composer.json
index fff8c67..a43a1e4 100644
--- a/composer.json
+++ b/composer.json
@@ -23,7 +23,7 @@
 		"php" : ">=7.1"
     },
 	"require-dev" : {
-		"phpunit/phpunit" : "^7.0",
+		"phpunit/phpunit" : "^9.0",
 		"php-coveralls/php-coveralls": "^2.1"
 	},
 	"autoload" : {
diff --git a/src/ArrObj.php b/src/ArrObj.php
index 6925302..b91796c 100644
--- a/src/ArrObj.php
+++ b/src/ArrObj.php
@@ -274,35 +274,58 @@ public function setArray($array): self
         return $this;
     }
 
-    public function offsetExists($offset)
+    /**
+     * @param mixed $offset
+     * @return bool
+     */
+    public function offsetExists($offset): bool
     {
         return array_key_exists($offset, $this->array);
     }
 
-    public function offsetGet($offset)
+    /**
+     * @param mixed $offset
+     * @return mixed|null
+     */
+    #[\ReturnTypeWillChange] public function offsetGet($offset)
     {
         return $this->array[$offset] ?? null;
     }
 
-    public function offsetSet($offset, $value)
+    /**
+     * @param mixed $offset
+     * @param mixed $value
+     * @return void
+     */
+    public function offsetSet($offset, $value): void
     {
-        return $this->array[$offset] = $value;
+        $this->array[$offset] = $value;
     }
 
-    public function offsetUnset($offset)
+    /**
+     * @param mixed $offset
+     * @return void
+     */
+    public function offsetUnset($offset): void
     {
         if (isset($this->array[$offset])) {
             unset($this->array[$offset]);
         }
     }
 
-    public function getIterator()
+    /**
+     * @return ArrayIterator
+     */
+    public function getIterator(): ArrayIterator
     {
         return new ArrayIterator($this->array);
     }
 
-    public function count()
+    /**
+     * @return int
+     */
+    public function count(): int
     {
         return count($this->array);
     }
-}
\ No newline at end of file
+}
diff --git a/tests/ArrObj/ArrObjTest.php b/tests/ArrObj/ArrObjTest.php
index 91d0342..71117df 100644
--- a/tests/ArrObj/ArrObjTest.php
+++ b/tests/ArrObj/ArrObjTest.php
@@ -13,7 +13,7 @@ class ArrObjTest extends ArrTestCase
     /** @var ArrObj */
     private $obj;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->obj = new ArrObj();
     }