diff --git a/composer.json b/composer.json
index 2cd9539..c19ff16 100644
--- a/composer.json
+++ b/composer.json
@@ -5,7 +5,7 @@
 
     "minimum-stability": "stable",
     "require":           {
-        "php": "^7.4 || ^8.0"
+        "php": "^8.1"
     },
     "autoload":          {
         "psr-4":                 {
diff --git a/src/Collections/Collection.php b/src/Collections/Collection.php
index 509a167..ea98db6 100644
--- a/src/Collections/Collection.php
+++ b/src/Collections/Collection.php
@@ -14,7 +14,7 @@ class Collection extends ImmutableCollection implements ICollection {
     /**
      * @inheritDoc
      */
-    public function set ($key, $value): self {
+    public function set (int|string $key, mixed $value): self {
         return $this->_set($key, $value);
     }
     /**
@@ -27,7 +27,7 @@ class Collection extends ImmutableCollection implements ICollection {
     /**
      * @inheritDoc
      */
-    public function add (...$values): self {
+    public function add (mixed ...$values): self {
         return $this->_add(...$values);
     }
     /**
@@ -40,7 +40,7 @@ class Collection extends ImmutableCollection implements ICollection {
     /**
      * @inheritDoc
      */
-    public function prepend (...$values): ICollection {
+    public function prepend (mixed ...$values): ICollection {
         array_unshift($this->elements, ...$values);
         return $this;
     }
@@ -69,14 +69,14 @@ class Collection extends ImmutableCollection implements ICollection {
     /**
      * @inheritDoc
      */
-    public function remove ($key): self {
+    public function remove (int|string $key): self {
         unset($this->elements[$this->_normalizeKey($key)]);
         return $this;
     }
     /**
      * @inheritDoc
      */
-    public function removeValue ($value, bool $strict = false): self {
+    public function removeValue (mixed $value, bool $strict = false): self {
         foreach ($this->elements as $currentKey => $currentValue) {
             if (($strict && $value === $currentValue) || (!$strict && $value == $currentValue)) {
                 unset($this->elements[$currentKey]);
@@ -88,13 +88,13 @@ class Collection extends ImmutableCollection implements ICollection {
     /**
      * @inheritDoc
      */
-    public function offsetSet ($offset, $value): void {
+    public function offsetSet (mixed $offset, mixed $value): void {
         $this->set($offset, $value);
     }
     /**
      * @inheritDoc
      */
-    public function offsetUnset ($offset): void {
+    public function offsetUnset (mixed $offset): void {
         $this->remove($offset);
     }
 }
\ No newline at end of file
diff --git a/src/Collections/ICollection.php b/src/Collections/ICollection.php
index 857e62c..cabb3b3 100644
--- a/src/Collections/ICollection.php
+++ b/src/Collections/ICollection.php
@@ -21,7 +21,7 @@ interface ICollection extends IImmutableCollection {
      *
      * @return $this
      */
-    public function set ($key, $value): self;
+    public function set (int|string $key, mixed $value): self;
     /**
      * Merge one or multiple collections in the current one
      *
@@ -40,7 +40,7 @@ interface ICollection extends IImmutableCollection {
      *
      * @return $this
      */
-    public function add (...$values): self;
+    public function add (mixed ...$values): self;
     /**
      * Add the <b>values</b> of one or multiple collections
      *
@@ -59,7 +59,7 @@ interface ICollection extends IImmutableCollection {
      *
      * @return $this
      */
-    public function prepend (...$values): self;
+    public function prepend (mixed ...$values): self;
     /**
      * Add the <b>values</b> of one or multiple collections at the beginning of the current collection
      *
@@ -84,7 +84,7 @@ interface ICollection extends IImmutableCollection {
      *
      * @return $this
      */
-    public function remove ($key): self;
+    public function remove (int|string $key): self;
     /**
      * Delete all instances of a value
      *
@@ -95,5 +95,5 @@ interface ICollection extends IImmutableCollection {
      *
      * @return $this
      */
-    public function removeValue ($value, bool $strict = false): self;
+    public function removeValue (mixed $value, bool $strict = false): self;
 }
\ No newline at end of file
diff --git a/src/Collections/IImmutableCollection.php b/src/Collections/IImmutableCollection.php
index 45a74ea..6bc3bc6 100644
--- a/src/Collections/IImmutableCollection.php
+++ b/src/Collections/IImmutableCollection.php
@@ -7,7 +7,6 @@ use Closure;
 use Countable;
 use IteratorAggregate;
 use JsonSerializable;
-use Serializable;
 
 /**
  * Interface for an immutable (read-only) collection
@@ -18,7 +17,7 @@ use Serializable;
  * @template-extends ArrayAccess<TKey, TValue>
  * @template-extends IArrayCast<TKey, TValue>
  */
-interface IImmutableCollection extends IteratorAggregate, JsonSerializable, Serializable, Countable, ArrayAccess, IArrayCast {
+interface IImmutableCollection extends IteratorAggregate, JsonSerializable, Countable, ArrayAccess, IArrayCast {
     /**
      * Checks if the collection is empty
      *
@@ -35,7 +34,7 @@ interface IImmutableCollection extends IteratorAggregate, JsonSerializable, Seri
      *
      * @return bool TRUE if the key exists, FALSE otherwise
      */
-    public function exists ($key): bool;
+    public function exists (int|string $key): bool;
     /**
      * Checks if contains a value
      *
@@ -46,7 +45,7 @@ interface IImmutableCollection extends IteratorAggregate, JsonSerializable, Seri
      *
      * @return bool TRUE if the value exists, FALSE otherwise
      */
-    public function contains ($value, bool $strict = false): bool;
+    public function contains (mixed $value, bool $strict = false): bool;
 
     /**
      * Get the value of a key or null if not found
@@ -58,7 +57,7 @@ interface IImmutableCollection extends IteratorAggregate, JsonSerializable, Seri
      * @return mixed The value
      * @psalm-return TValue|null
      */
-    public function get ($key);
+    public function get (int|string $key): mixed;
     /**
      * Get the first key of a value or null if not found
      *
@@ -67,10 +66,10 @@ interface IImmutableCollection extends IteratorAggregate, JsonSerializable, Seri
      *
      * @psalm-param TValue $value
      *
-     * @return array-key
+     * @return array-key|null
      * @psalm-return TKey|null
      */
-    public function key ($value, bool $strict = false);
+    public function key (mixed $value, bool $strict = false): int|string|null;
 
     /**
      * Extract a slice of the collection
diff --git a/src/Collections/ImmutableCollection.php b/src/Collections/ImmutableCollection.php
index e358d87..d1e8874 100644
--- a/src/Collections/ImmutableCollection.php
+++ b/src/Collections/ImmutableCollection.php
@@ -5,7 +5,6 @@ namespace jrosset\Collections;
 use ArrayIterator;
 use Closure;
 use InvalidArgumentException;
-use JsonException;
 use Traversable;
 
 /**
@@ -27,28 +26,27 @@ class ImmutableCollection implements IImmutableCollection {
     /**
      * Initialise a new collection
      *
-     * @param array|Traversable|null $other The initial values
+     * @param iterable|null                                            $other The initial values
      *
-     * @psalm-param array<TKey, TValue>|Traversable<TKey, TValue>|null
+     * @psalm-param array<TKey, TValue>|Traversable<TKey, TValue>|null $other
      *
      * @throws InvalidArgumentException If the initial values aren't valid
      */
-    public function __construct ($other = null) {
+    public function __construct (?iterable $other = null) {
         $this->_initialize($other);
     }
 
     /**
      * Initialise the internals elements
      *
-     * @param array|Traversable|null $other The initial values
+     * @param iterable|null                                            $other The initial values
      *
-     * @psalm-param array<TKey, TValue>|Traversable<TKey, TValue>|null
+     * @psalm-param array<TKey, TValue>|Traversable<TKey, TValue>|null $other
      *
      * @throws InvalidArgumentException If the initial values aren't valid
      * @internal
-     *
      */
-    protected function _initialize ($other = null) {
+    protected function _initialize (?iterable $other = null) {
         $this->elements = [];
         if ($other !== null) {
             if (!is_array($other) && !$other instanceof Traversable) {
@@ -73,7 +71,7 @@ class ImmutableCollection implements IImmutableCollection {
      * @internal
      *
      */
-    protected function _set ($key, $value): self {
+    protected function _set (int|string $key, mixed $value): self {
         $this->elements[$this->_normalizeKey($key)] = $value;
         return $this;
     }
@@ -103,7 +101,7 @@ class ImmutableCollection implements IImmutableCollection {
      * @internal
      *
      */
-    protected function _add (...$values): self {
+    protected function _add (mixed ...$values): self {
         foreach ($values as $value) {
             $this->elements[] = $value;
         }
@@ -134,53 +132,49 @@ class ImmutableCollection implements IImmutableCollection {
      * @return array-key The normalized key
      * @psalm-return TKey
      */
-    protected function _normalizeKey ($key) {
+    protected function _normalizeKey (int|string $key): int|string {
         return $key;
     }
 
     /**
      * @inheritDoc
      */
-    public function getIterator () {
+    public function getIterator (): ArrayIterator {
         return new ArrayIterator($this->elements);
     }
 
     /**
      * @inheritDoc
      */
-    public function offsetExists ($offset): bool {
+    public function offsetExists (mixed $offset): bool {
         return $this->exists($offset);
     }
     /**
      * @inheritDoc
      */
-    public function offsetGet ($offset) {
+    public function offsetGet (mixed $offset): mixed {
         return $this->get($offset);
     }
     /**
      * @inheritDoc
      */
-    public function offsetSet ($offset, $value): void {
+    public function offsetSet (mixed $offset, mixed $value): void {
         throw new ImmutableException();
     }
     /**
      * @inheritDoc
      */
-    public function offsetUnset ($offset): void {
+    public function offsetUnset (mixed $offset): void {
         throw new ImmutableException();
     }
 
-    /**
-     * @inheritDoc
-     */
-    public function serialize (): ?string {
-        return serialize($this->elements);
+    public function __serialize (): array {
+        return [
+            'elements' => $this->elements,
+        ];
     }
-    /**
-     * @inheritDoc
-     */
-    public function unserialize ($data) {
-        $this->_initialize(unserialize($data));
+    public function __unserialize (array $data): void {
+        $this->_initialize($data['elements']);
     }
 
     /**
@@ -206,26 +200,26 @@ class ImmutableCollection implements IImmutableCollection {
     /**
      * @inheritDoc
      */
-    public function exists ($key): bool {
+    public function exists (int|string $key): bool {
         return isset($this->elements[$this->_normalizeKey($key)]);
     }
     /**
      * @inheritDoc
      */
-    public function contains ($value, bool $strict = false): bool {
+    public function contains (mixed $value, bool $strict = false): bool {
         return in_array($value, $this->elements, $strict);
     }
 
     /**
      * @inheritDoc
      */
-    public function get ($key) {
+    public function get (int|string $key): mixed {
         return $this->elements[$this->_normalizeKey($key)] ?? null;
     }
     /**
      * @inheritDoc
      */
-    public function key ($value, bool $strict = false) {
+    public function key (mixed $value, bool $strict = false): int|string|null {
         foreach ($this->elements as $currentKey => $currentValue) {
             if (($strict && $value === $currentValue) || (!$strict && $value == $currentValue)) {
                 return $currentKey;
@@ -309,11 +303,9 @@ class ImmutableCollection implements IImmutableCollection {
 
     /**
      * @inheritDoc
-     *
-     * @throws JsonException
      */
-    public function jsonSerialize () {
-        return json_encode($this->elements, JSON_THROW_ON_ERROR);
+    public function jsonSerialize (): mixed {
+        return $this->elements;
     }
 
     /**
diff --git a/src/Collections/TInsensitiveCaseKey.php b/src/Collections/TInsensitiveCaseKey.php
index d2d4757..9e1f794 100644
--- a/src/Collections/TInsensitiveCaseKey.php
+++ b/src/Collections/TInsensitiveCaseKey.php
@@ -7,13 +7,9 @@ namespace jrosset\Collections;
  */
 trait TInsensitiveCaseKey {
     /**
-     * Normalize a key
-     *
-     * @param array-key $key The key to normalize
-     *
-     * @return array-key The normalized key
+     * @inheritDoc
      */
-    protected function _normalizeKey ($key) {
+    protected function _normalizeKey (int|string $key): int|string {
         return mb_strtolower($key);
     }
 }
\ No newline at end of file