diff --git a/.github/workflows/continuous-integration.yml b/.github/workflows/continuous-integration.yml
index e56dd68f..da459b99 100644
--- a/.github/workflows/continuous-integration.yml
+++ b/.github/workflows/continuous-integration.yml
@@ -16,13 +16,21 @@ jobs:
   lint:
     name: "Lint"
     runs-on: "ubuntu-latest"
+
+    strategy:
+      fail-fast: false
+      matrix:
+        php-version: [ '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1' ]
+
+    continue-on-error: ${{ matrix.php-version == '8.1' }}
+
     steps:
       - uses: "actions/checkout@v2"
       - uses: "shivammathur/setup-php@v2"
         with:
           php-version: "7.4"
           coverage: "none"
-          ini-values: "memory_limit=-1"
+          ini-values: "memory_limit=-1, zend.assertions=1, error_reporting=-1, display_errors=On"
           tools: "composer:v2"
       - uses: "ramsey/composer-install@v1"
       - name: "Lint the PHP source code"
@@ -58,19 +66,17 @@ jobs:
           - "7.3"
           - "7.4"
           - "8.0"
+          - "8.1"
         experimental:
           - false
-        include:
-          - php-version: "8.1"
-            experimental: true
-            composer-options: "--ignore-platform-reqs"
+
     steps:
       - uses: "actions/checkout@v2"
       - uses: "shivammathur/setup-php@v2"
         with:
           php-version: "${{ matrix.php-version }}"
           coverage: "pcov"
-          ini-values: "memory_limit=-1"
+          ini-values: "memory_limit=-1, zend.assertions=1, error_reporting=-1, display_errors=On"
           tools: "composer:v2"
       - name: "Prepare for tests"
         run: "mkdir -p build/logs"
diff --git a/.gitignore b/.gitignore
index 8ef05678..9d85098e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@ testing/
 nbproject/private/
 test/log
 build
+/.phpunit.result.cache
diff --git a/composer.json b/composer.json
index 321a2c26..59201f48 100644
--- a/composer.json
+++ b/composer.json
@@ -11,9 +11,9 @@
         "paragonie/random_compat": "^1 || ^2 || ^9.99"
     },
     "require-dev": {
-        "mockery/mockery": "^1.3",
-        "php-parallel-lint/php-parallel-lint": "^1.2",
-        "phpunit/phpunit": "^5.7 || ^6.0 || ^9.3",
+        "mockery/mockery": "^1.3.5",
+        "php-parallel-lint/php-parallel-lint": "^1.3.1",
+        "phpunit/phpunit": "^5.7 || ^6.0 || ^9.5",
         "squizlabs/php_codesniffer": "^2.3 || ^3.0"
     },
     "keywords": [
diff --git a/phpunit.xml b/phpunit.xml
index bf749bfb..46accb38 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -1,5 +1,12 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit colors="true" bootstrap="./vendor/autoload.php">
+<phpunit
+    bootstrap="./vendor/autoload.php"
+    colors="true"
+    convertErrorsToExceptions="true"
+    convertWarningsToExceptions="true"
+    convertNoticesToExceptions="true"
+    convertDeprecationsToExceptions="true"
+>
     <testsuites>
         <testsuite name="all">
             <directory>./test</directory>
diff --git a/src/Token/AccessTokenInterface.php b/src/Token/AccessTokenInterface.php
index c5f13350..5fd219ff 100644
--- a/src/Token/AccessTokenInterface.php
+++ b/src/Token/AccessTokenInterface.php
@@ -15,6 +15,7 @@
 namespace League\OAuth2\Client\Token;
 
 use JsonSerializable;
+use ReturnTypeWillChange;
 use RuntimeException;
 
 interface AccessTokenInterface extends JsonSerializable
@@ -68,5 +69,6 @@ public function __toString();
      *
      * @return array
      */
+    #[ReturnTypeWillChange]
     public function jsonSerialize();
 }
diff --git a/src/Tool/QueryBuilderTrait.php b/src/Tool/QueryBuilderTrait.php
index ebccdffc..bdda3e79 100644
--- a/src/Tool/QueryBuilderTrait.php
+++ b/src/Tool/QueryBuilderTrait.php
@@ -28,6 +28,6 @@ trait QueryBuilderTrait
      */
     protected function buildQueryString(array $params)
     {
-        return http_build_query($params, null, '&', \PHP_QUERY_RFC3986);
+        return http_build_query($params, '', '&', \PHP_QUERY_RFC3986);
     }
 }