diff --git a/src/Optimizely/DecisionService/DecisionService.php b/src/Optimizely/DecisionService/DecisionService.php
index cb8f6bd6..6b7d9259 100644
--- a/src/Optimizely/DecisionService/DecisionService.php
+++ b/src/Optimizely/DecisionService/DecisionService.php
@@ -81,9 +81,9 @@ class DecisionService
      * DecisionService constructor.
      *
      * @param LoggerInterface       $logger
-     * @param UserProfileServiceInterface  $userProfileService
+     * @param ?UserProfileServiceInterface  $userProfileService
      */
-    public function __construct(LoggerInterface $logger, UserProfileServiceInterface $userProfileService = null)
+    public function __construct(LoggerInterface $logger, ?UserProfileServiceInterface $userProfileService = null)
     {
         $this->_logger = $logger;
         $this->_bucketer = new Bucketer($logger);
diff --git a/src/Optimizely/Event/Dispatcher/DefaultEventDispatcher.php b/src/Optimizely/Event/Dispatcher/DefaultEventDispatcher.php
index b5b4b7c3..a45a1074 100644
--- a/src/Optimizely/Event/Dispatcher/DefaultEventDispatcher.php
+++ b/src/Optimizely/Event/Dispatcher/DefaultEventDispatcher.php
@@ -36,7 +36,7 @@ class DefaultEventDispatcher implements EventDispatcherInterface
      */
     private $httpClient;
 
-    public function __construct(HttpClient $httpClient = null)
+    public function __construct(?HttpClient $httpClient = null)
     {
         $this->httpClient = $httpClient ?: new HttpClient();
     }
diff --git a/src/Optimizely/Optimizely.php b/src/Optimizely/Optimizely.php
index 69833f56..968e1a0a 100644
--- a/src/Optimizely/Optimizely.php
+++ b/src/Optimizely/Optimizely.php
@@ -119,24 +119,24 @@ class Optimizely
      * Optimizely constructor for managing Feature Experimentation PHP projects.
      *
      * @param $datafile string JSON string representing the project.
-     * @param $eventDispatcher EventDispatcherInterface
-     * @param $logger LoggerInterface
-     * @param $errorHandler ErrorHandlerInterface
+     * @param $eventDispatcher ?EventDispatcherInterface
+     * @param $logger ?LoggerInterface
+     * @param $errorHandler ?ErrorHandlerInterface
      * @param $skipJsonValidation boolean representing whether JSON schema validation needs to be performed.
-     * @param $userProfileService UserProfileServiceInterface
-     * @param $configManager ProjectConfigManagerInterface provides ProjectConfig through getConfig method.
-     * @param $notificationCenter NotificationCenter
+     * @param $userProfileService ?UserProfileServiceInterface
+     * @param $configManager ?ProjectConfigManagerInterface provides ProjectConfig through getConfig method.
+     * @param $notificationCenter ?NotificationCenter
      * @param $sdkKey string uniquely identifying the datafile corresponding to project and environment combination. Must provide at least one of datafile or sdkKey.
      */
     public function __construct(
         $datafile,
-        EventDispatcherInterface $eventDispatcher = null,
-        LoggerInterface $logger = null,
-        ErrorHandlerInterface $errorHandler = null,
+        ?EventDispatcherInterface $eventDispatcher = null,
+        ?LoggerInterface $logger = null,
+        ?ErrorHandlerInterface $errorHandler = null,
         $skipJsonValidation = false,
-        UserProfileServiceInterface $userProfileService = null,
-        ProjectConfigManagerInterface $configManager = null,
-        NotificationCenter $notificationCenter = null,
+        ?UserProfileServiceInterface $userProfileService = null,
+        ?ProjectConfigManagerInterface $configManager = null,
+        ?NotificationCenter $notificationCenter = null,
         $sdkKey = null,
         array $defaultDecideOptions = []
     ) {
diff --git a/src/Optimizely/OptimizelyConfig/OptimizelyConfigService.php b/src/Optimizely/OptimizelyConfig/OptimizelyConfigService.php
index c8d22c36..d6463ee2 100644
--- a/src/Optimizely/OptimizelyConfig/OptimizelyConfigService.php
+++ b/src/Optimizely/OptimizelyConfig/OptimizelyConfigService.php
@@ -85,7 +85,7 @@ class OptimizelyConfigService
 
     private ProjectConfigInterface $projectConfig;
 
-    public function __construct(ProjectConfigInterface $projectConfig, LoggerInterface $logger = null)
+    public function __construct(ProjectConfigInterface $projectConfig, ?LoggerInterface $logger = null)
     {
         $this->experiments = $projectConfig->getAllExperiments();
         $this->featureFlags = $projectConfig->getFeatureFlags();
diff --git a/src/Optimizely/ProjectConfigManager/HTTPProjectConfigManager.php b/src/Optimizely/ProjectConfigManager/HTTPProjectConfigManager.php
index 83e5f839..5e56935e 100644
--- a/src/Optimizely/ProjectConfigManager/HTTPProjectConfigManager.php
+++ b/src/Optimizely/ProjectConfigManager/HTTPProjectConfigManager.php
@@ -89,9 +89,9 @@ public function __construct(
         $fetchOnInit = true,
         $datafile = null,
         $skipJsonValidation = false,
-        LoggerInterface $logger = null,
-        ErrorHandlerInterface $errorHandler = null,
-        NotificationCenter $notificationCenter = null,
+        ?LoggerInterface $logger = null,
+        ?ErrorHandlerInterface $errorHandler = null,
+        ?NotificationCenter $notificationCenter = null,
         $datafileAccessToken = null
     ) {
         $this->_skipJsonValidation = $skipJsonValidation;
diff --git a/src/Optimizely/Utils/Validator.php b/src/Optimizely/Utils/Validator.php
index ff2f1c5b..803bc25b 100644
--- a/src/Optimizely/Utils/Validator.php
+++ b/src/Optimizely/Utils/Validator.php
@@ -31,7 +31,7 @@ class Validator
      *
      * @return boolean Representing whether schema is valid or not.
      */
-    public static function validateJsonSchema($datafile, LoggerInterface $logger = null)
+    public static function validateJsonSchema($datafile, ?LoggerInterface $logger = null)
     {
         $data = json_decode($datafile);
 
diff --git a/src/Optimizely/Utils/VariableTypeUtils.php b/src/Optimizely/Utils/VariableTypeUtils.php
index a6e534ef..faa9a1ea 100644
--- a/src/Optimizely/Utils/VariableTypeUtils.php
+++ b/src/Optimizely/Utils/VariableTypeUtils.php
@@ -24,7 +24,7 @@
 
 class VariableTypeUtils
 {
-    public static function castStringToType($value, $variableType, LoggerInterface $logger = null)
+    public static function castStringToType($value, $variableType, ?LoggerInterface $logger = null)
     {
         if ($variableType == FeatureVariable::STRING_TYPE) {
             return $value;