diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index ccd13c23..aa1036fa 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -14,7 +14,7 @@ jobs:
       matrix:
 #        os: [ubuntu-latest, macos-latest, windows-latest]
         os: [ubuntu-latest]
-        php: ['7.1', '7.2', '7.3', '7.4', '8.0']
+        php: ['7.1', '7.2', '7.3', '7.4', '8.0', '8.1']
         # max 4.4.16, see https://github.com/symfony/symfony/issues/39521
         # max 5.1.8, see https://github.com/symfony/symfony/issues/39521
         yaml: ['5.2.9', '5.1.11', '4.4.24', '^3.4']
diff --git a/src/json/JsonReference.php b/src/json/JsonReference.php
index fe27ced1..0550cad4 100644
--- a/src/json/JsonReference.php
+++ b/src/json/JsonReference.php
@@ -8,6 +8,7 @@
 namespace cebe\openapi\json;
 
 use JsonSerializable;
+use stdClass;
 
 /**
  * Represents a JSON Reference (IETF draft-pbryan-zyp-json-ref-03)
@@ -123,10 +124,8 @@ public function getReference(): string
     /**
      * Specify data which should be serialized to JSON
      * @link https://php.net/manual/en/jsonserializable.jsonserialize.php
-     * @return mixed data which can be serialized by <b>json_encode</b>,
-     * which is a value of any type other than a resource.
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): stdClass
     {
         return (object)['$ref' => $this->getReference()];
     }
diff --git a/src/spec/Paths.php b/src/spec/Paths.php
index a2da8a10..7a04ba48 100644
--- a/src/spec/Paths.php
+++ b/src/spec/Paths.php
@@ -180,10 +180,9 @@ public function getErrors(): array
      * Whether a offset exists
      * @link http://php.net/manual/en/arrayaccess.offsetexists.php
      * @param mixed $offset An offset to check for.
-     * @return boolean true on success or false on failure.
      * The return value will be casted to boolean if non-boolean was returned.
      */
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return $this->hasPath($offset);
     }
@@ -192,9 +191,8 @@ public function offsetExists($offset)
      * Offset to retrieve
      * @link http://php.net/manual/en/arrayaccess.offsetget.php
      * @param mixed $offset The offset to retrieve.
-     * @return PathItem Can return all value types.
      */
-    public function offsetGet($offset)
+    public function offsetGet($offset): ?PathItem
     {
         return $this->getPath($offset);
     }
@@ -205,7 +203,7 @@ public function offsetGet($offset)
      * @param mixed $offset The offset to assign the value to.
      * @param mixed $value The value to set.
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         $this->addPath($offset, $value);
     }
@@ -215,7 +213,7 @@ public function offsetSet($offset, $value)
      * @link http://php.net/manual/en/arrayaccess.offsetunset.php
      * @param mixed $offset The offset to unset.
      */
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         $this->removePath($offset);
     }
@@ -226,7 +224,7 @@ public function offsetUnset($offset)
      * @return int The custom count as an integer.
      * The return value is cast to an integer.
      */
-    public function count()
+    public function count(): int
     {
         return count($this->_paths);
     }
@@ -234,9 +232,8 @@ public function count()
     /**
      * Retrieve an external iterator
      * @link http://php.net/manual/en/iteratoraggregate.getiterator.php
-     * @return Traversable An instance of an object implementing <b>Iterator</b> or <b>Traversable</b>
      */
-    public function getIterator()
+    public function getIterator(): Traversable
     {
         return new ArrayIterator($this->_paths);
     }
diff --git a/src/spec/Responses.php b/src/spec/Responses.php
index 52476031..e0367015 100644
--- a/src/spec/Responses.php
+++ b/src/spec/Responses.php
@@ -170,10 +170,9 @@ public function getErrors(): array
      * Whether a offset exists
      * @link http://php.net/manual/en/arrayaccess.offsetexists.php
      * @param mixed $offset An offset to check for.
-     * @return boolean true on success or false on failure.
      * The return value will be casted to boolean if non-boolean was returned.
      */
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return $this->hasResponse($offset);
     }
@@ -182,9 +181,8 @@ public function offsetExists($offset)
      * Offset to retrieve
      * @link http://php.net/manual/en/arrayaccess.offsetget.php
      * @param mixed $offset The offset to retrieve.
-     * @return mixed Can return all value types.
      */
-    public function offsetGet($offset)
+    public function offsetGet($offset): ?SpecObjectInterface
     {
         return $this->getResponse($offset);
     }
@@ -195,7 +193,7 @@ public function offsetGet($offset)
      * @param mixed $offset The offset to assign the value to.
      * @param mixed $value The value to set.
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         $this->addResponse($offset, $value);
     }
@@ -205,7 +203,7 @@ public function offsetSet($offset, $value)
      * @link http://php.net/manual/en/arrayaccess.offsetunset.php
      * @param mixed $offset The offset to unset.
      */
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         $this->removeResponse($offset);
     }
@@ -216,7 +214,7 @@ public function offsetUnset($offset)
      * @return int The custom count as an integer.
      * The return value is cast to an integer.
      */
-    public function count()
+    public function count(): int
     {
         return count($this->_responses);
     }
@@ -226,7 +224,7 @@ public function count()
      * @link http://php.net/manual/en/iteratoraggregate.getiterator.php
      * @return Traversable An instance of an object implementing <b>Iterator</b> or <b>Traversable</b>
      */
-    public function getIterator()
+    public function getIterator(): Traversable
     {
         return new ArrayIterator($this->_responses);
     }