diff --git a/composer.json b/composer.json
index cf3a639..f421443 100644
--- a/composer.json
+++ b/composer.json
@@ -26,14 +26,15 @@
   "require": {
     "php": "^7.4.0||^8.0",
     "guzzlehttp/guzzle": "^7.2",
-    "ext-json": "*"
+    "ext-json": "*",
+    "ext-mbstring": "*"
   },
   "require-dev": {
     "dealerdirect/phpcodesniffer-composer-installer": "^0.7",
     "php-vcr/php-vcr": "^1.5.2",
     "phpstan/phpstan": "^1.10",
+    "phpstan/phpstan-deprecation-rules": "^1.1",
     "phpunit/phpunit": "^9.0",
-    "symfony/var-dumper": "^4.2",
     "vlucas/phpdotenv": "^2.5"
   },
   "autoload": {
diff --git a/phpstan.neon b/phpstan.neon
index 228341e..d8e1234 100755
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -1,5 +1,6 @@
 includes:
     - phpstan-baseline.neon
+    - vendor/phpstan/phpstan-deprecation-rules/rules.neon
 
 parameters:
     paths:
diff --git a/src/Middleware/AuthMiddleware.php b/src/Middleware/AuthMiddleware.php
index b337afe..ca8200b 100644
--- a/src/Middleware/AuthMiddleware.php
+++ b/src/Middleware/AuthMiddleware.php
@@ -109,7 +109,7 @@ private function makeSignatureString(string $signatureContentString)
             hash_hmac(
                 static::SIGNATURE_ALGORITHM,
                 $signatureContentString,
-                utf8_encode($this->apiSecret),
+                mb_convert_encoding($this->apiSecret, 'UTF-8', mb_detect_encoding($this->apiSecret)),
                 true
             )
         );
diff --git a/src/XCover.php b/src/XCover.php
index 55e6222..032513b 100644
--- a/src/XCover.php
+++ b/src/XCover.php
@@ -5,8 +5,8 @@
 use GuzzleHttp\Client;
 use GuzzleHttp\ClientInterface;
 use GuzzleHttp\Exception\GuzzleException;
+use GuzzleHttp\Utils;
 use GuzzleHttp\HandlerStack;
-use Psr\Http\Message\ResponseInterface;
 use XCoverClient\Exceptions\ResponseException;
 use XCoverClient\Exceptions\XCoverException;
 use XCoverClient\Middleware\AuthMiddleware;
@@ -50,7 +50,7 @@ public function __construct(Config $config, ClientInterface $client = null)
     public function call($method, $url, $expectedStatusCode = null, $payload = [], $queryParams = [])
     {
         $options = [
-            'body' => $payload ? \GuzzleHttp\json_encode($payload) : "{}",
+            'body' => $payload ? Utils::jsonEncode($payload) : "{}",
             'query' => $queryParams,
         ];