diff --git a/CHANGELOG.md b/CHANGELOG.md
index c0f80d0b9..76eb8536c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ Starting with version 5, the Facebook PHP SDK follows [SemVer](http://semver.org
 
 Version 5 of the Facebook PHP SDK is a complete refactor of version 4. It comes loaded with lots of new features and a friendlier API.
 
+- 5.9.0 (2022-01-20) Support php8.1
 - 5.7.1 (2018-XX-XX)
 - 5.7.0 (2018-12-12)
   - Add `joined` to list of fields to be cast to `\DateTime` (#950)
diff --git a/composer.json b/composer.json
index 9d54abc37..9a0313fce 100644
--- a/composer.json
+++ b/composer.json
@@ -12,7 +12,7 @@
         }
     ],
     "require": {
-        "php": "^5.4|^7.0"
+        "php": "^5.4|^7.0|^8.0"
     },
     "require-dev": {
         "phpunit/phpunit": "~4.0",
diff --git a/src/Facebook/Authentication/OAuth2Client.php b/src/Facebook/Authentication/OAuth2Client.php
index 94df9b7b5..3ba05d80a 100644
--- a/src/Facebook/Authentication/OAuth2Client.php
+++ b/src/Facebook/Authentication/OAuth2Client.php
@@ -143,7 +143,7 @@ public function getAuthorizationUrl($redirectUrl, $state, array $scope = [], arr
             'scope' => implode(',', $scope)
         ];
 
-        return static::BASE_AUTHORIZATION_URL . '/' . $this->graphVersion . '/dialog/oauth?' . http_build_query($params, null, $separator);
+        return static::BASE_AUTHORIZATION_URL . '/' . $this->graphVersion . '/dialog/oauth?' . http_build_query($params, '', $separator);
     }
 
     /**
diff --git a/src/Facebook/FacebookApp.php b/src/Facebook/FacebookApp.php
index 804c9bb56..a7a962ab7 100644
--- a/src/Facebook/FacebookApp.php
+++ b/src/Facebook/FacebookApp.php
@@ -47,8 +47,8 @@ class FacebookApp implements \Serializable
     public function __construct($id, $secret)
     {
         if (!is_string($id)
-          // Keeping this for BC. Integers greater than PHP_INT_MAX will make is_int() return false
-          && !is_int($id)) {
+            // Keeping this for BC. Integers greater than PHP_INT_MAX will make is_int() return false
+            && !is_int($id)) {
             throw new FacebookSDKException('The "app_id" must be formatted as a string since many app ID\'s are greater than PHP_INT_MAX on some systems.');
         }
         // We cast as a string in case a valid int was set on a 64-bit system and this is unserialised on a 32-bit system
@@ -91,9 +91,9 @@ public function getAccessToken()
      *
      * @return string
      */
-    public function serialize()
+    public function serialize(): string
     {
-        return implode('|', [$this->id, $this->secret]);
+        return implode('|', $this->__serialize());
     }
 
     /**
@@ -101,10 +101,25 @@ public function serialize()
      *
      * @param string $serialized
      */
-    public function unserialize($serialized)
+    public function unserialize(string $serialized): void
     {
-        list($id, $secret) = explode('|', $serialized);
+        $this->__unserialize(explode('|', $serialized));
+    }
 
+    /**
+     * @internal
+     */
+    public function __serialize(): array
+    {
+        return [$this->id, $this->secret];
+    }
+
+    /**
+     * @internal
+     */
+    public function __unserialize(array $data): void
+    {
+        list($id, $secret) = $data;
         $this->__construct($id, $secret);
     }
 }
diff --git a/src/Facebook/Http/RequestBodyMultipart.php b/src/Facebook/Http/RequestBodyMultipart.php
index e43695a4f..949080e9c 100644
--- a/src/Facebook/Http/RequestBodyMultipart.php
+++ b/src/Facebook/Http/RequestBodyMultipart.php
@@ -144,7 +144,7 @@ private function getParamString($name, $value)
      */
     private function getNestedParams(array $params)
     {
-        $query = http_build_query($params, null, '&');
+        $query = http_build_query($params, '', '&');
         $params = explode('&', $query);
         $result = [];
 
diff --git a/src/Facebook/Http/RequestBodyUrlEncoded.php b/src/Facebook/Http/RequestBodyUrlEncoded.php
index c1e35f43d..dee58b9f8 100644
--- a/src/Facebook/Http/RequestBodyUrlEncoded.php
+++ b/src/Facebook/Http/RequestBodyUrlEncoded.php
@@ -50,6 +50,6 @@ public function __construct(array $params)
      */
     public function getBody()
     {
-        return http_build_query($this->params, null, '&');
+        return http_build_query($this->params, '', '&');
     }
 }
diff --git a/src/Facebook/Url/FacebookUrlManipulator.php b/src/Facebook/Url/FacebookUrlManipulator.php
index daeab9c52..490ed1d91 100644
--- a/src/Facebook/Url/FacebookUrlManipulator.php
+++ b/src/Facebook/Url/FacebookUrlManipulator.php
@@ -53,7 +53,7 @@ public static function removeParamsFromUrl($url, array $paramsToFilter)
             }
 
             if (count($params) > 0) {
-                $query = '?' . http_build_query($params, null, '&');
+                $query = '?' . http_build_query($params, '', '&');
             }
         }
 
@@ -81,7 +81,7 @@ public static function appendParamsToUrl($url, array $newParams = [])
         }
 
         if (strpos($url, '?') === false) {
-            return $url . '?' . http_build_query($newParams, null, '&');
+            return $url . '?' . http_build_query($newParams, '', '&');
         }
 
         list($path, $query) = explode('?', $url, 2);
@@ -94,7 +94,7 @@ public static function appendParamsToUrl($url, array $newParams = [])
         // Sort for a predicable order
         ksort($newParams);
 
-        return $path . '?' . http_build_query($newParams, null, '&');
+        return $path . '?' . http_build_query($newParams, '', '&');
     }
 
     /**