diff --git a/framework/base/Component.php b/framework/base/Component.php
index fb4805df0ce..f2eb9bf05c1 100644
--- a/framework/base/Component.php
+++ b/framework/base/Component.php
@@ -605,7 +605,7 @@ public function off($name, $handler = null)
      * @param string $name the event name
      * @param Event|null $event the event instance. If not set, a default [[Event]] object will be created.
      */
-    public function trigger($name, Event $event = null)
+    public function trigger($name, ?Event $event = null)
     {
         $this->ensureBehaviors();
 
diff --git a/framework/db/ActiveQuery.php b/framework/db/ActiveQuery.php
index 2153e7db155..8f5fc9903e1 100644
--- a/framework/db/ActiveQuery.php
+++ b/framework/db/ActiveQuery.php
@@ -785,7 +785,7 @@ public function orOnCondition($condition, $params = [])
      * @throws InvalidConfigException when query is not initialized properly
      * @see via()
      */
-    public function viaTable($tableName, $link, callable $callable = null)
+    public function viaTable($tableName, $link, ?callable $callable = null)
     {
         $modelClass = $this->primaryModel ? get_class($this->primaryModel) : $this->modelClass;
         $relation = new self($modelClass, [
diff --git a/framework/db/ActiveQueryInterface.php b/framework/db/ActiveQueryInterface.php
index ec53af2d4bc..f15e49eb008 100644
--- a/framework/db/ActiveQueryInterface.php
+++ b/framework/db/ActiveQueryInterface.php
@@ -97,7 +97,7 @@ public function with();
      * Its signature should be `function($query)`, where `$query` is the query to be customized.
      * @return $this the relation object itself.
      */
-    public function via($relationName, callable $callable = null);
+    public function via($relationName, ?callable $callable = null);
 
     /**
      * Finds the related records for the specified primary record.
diff --git a/framework/db/ActiveRelationTrait.php b/framework/db/ActiveRelationTrait.php
index 7e01e25fd99..467203e8a9f 100644
--- a/framework/db/ActiveRelationTrait.php
+++ b/framework/db/ActiveRelationTrait.php
@@ -104,7 +104,7 @@ public function __clone()
      * Its signature should be `function($query)`, where `$query` is the query to be customized.
      * @return $this the relation object itself.
      */
-    public function via($relationName, callable $callable = null)
+    public function via($relationName, ?callable $callable = null)
     {
         $relation = $this->primaryModel->getRelation($relationName);
         $callableUsed = $callable !== null;
diff --git a/framework/mail/BaseMessage.php b/framework/mail/BaseMessage.php
index ca53dac10c9..30940994499 100644
--- a/framework/mail/BaseMessage.php
+++ b/framework/mail/BaseMessage.php
@@ -39,7 +39,7 @@ abstract class BaseMessage extends BaseObject implements MessageInterface
      * the "mailer" application component will be used instead.
      * @return bool whether this message is sent successfully.
      */
-    public function send(MailerInterface $mailer = null)
+    public function send(?MailerInterface $mailer = null)
     {
         if ($mailer === null && $this->mailer === null) {
             $mailer = Yii::$app->getMailer();
diff --git a/framework/mail/MessageInterface.php b/framework/mail/MessageInterface.php
index 091dce5faa7..e4c62a2d528 100644
--- a/framework/mail/MessageInterface.php
+++ b/framework/mail/MessageInterface.php
@@ -209,7 +209,7 @@ public function embedContent($content, array $options = []);
      * If null, the "mailer" application component will be used instead.
      * @return bool whether this message is sent successfully.
      */
-    public function send(MailerInterface $mailer = null);
+    public function send(?MailerInterface $mailer = null);
 
     /**
      * Returns string representation of this message.
diff --git a/tests/framework/console/FakePhp71Controller.php b/tests/framework/console/FakePhp71Controller.php
index dbed836f8d1..cc23ae60299 100644
--- a/tests/framework/console/FakePhp71Controller.php
+++ b/tests/framework/console/FakePhp71Controller.php
@@ -14,7 +14,7 @@
 
 class FakePhp71Controller extends Controller
 {
-    public function actionInjection($before, Request $request, $between, DummyService $dummyService, Post $post = null, $after)
+    public function actionInjection($before, Request $request, $between, DummyService $dummyService, ?Post $post = null, $after)
     {
 
     }
diff --git a/tests/framework/di/ContainerTest.php b/tests/framework/di/ContainerTest.php
index ba62b598b73..2555f43c2d6 100644
--- a/tests/framework/di/ContainerTest.php
+++ b/tests/framework/di/ContainerTest.php
@@ -252,7 +252,7 @@ public function testOptionalDependencies()
     {
         $container = new Container();
         // Test optional unresolvable dependency.
-        $closure = function (QuxInterface $test = null) {
+        $closure = function (?QuxInterface $test = null) {
             return $test;
         };
         $this->assertNull($container->invoke($closure));
diff --git a/tests/framework/di/stubs/Alpha.php b/tests/framework/di/stubs/Alpha.php
index 151d2f109bb..a5af3ffed78 100644
--- a/tests/framework/di/stubs/Alpha.php
+++ b/tests/framework/di/stubs/Alpha.php
@@ -12,10 +12,10 @@ class Alpha extends BaseObject
     public $color = true;
 
     public function __construct(
-        Beta $beta = null,
-        QuxInterface $omega = null,
-        Unknown $unknown = null,
-        AbstractColor $color = null
+        ?Beta $beta = null,
+        ?QuxInterface $omega = null,
+        ?Unknown $unknown = null,
+        ?AbstractColor $color = null
     ) {
         $this->beta = $beta;
         $this->omega = $omega;
diff --git a/tests/framework/web/FakePhp71Controller.php b/tests/framework/web/FakePhp71Controller.php
index d8aa0e406c5..f84214c6e8d 100644
--- a/tests/framework/web/FakePhp71Controller.php
+++ b/tests/framework/web/FakePhp71Controller.php
@@ -21,7 +21,7 @@ class FakePhp71Controller extends Controller
 {
     public $enableCsrfValidation = false;
 
-    public function actionInjection($before, Request $request, $between, VendorImage $vendorImage, Post $post = null, $after)
+    public function actionInjection($before, Request $request, $between, VendorImage $vendorImage, ?Post $post = null, $after)
     {
 
     }
diff --git a/tests/framework/web/FakePhp7Controller.php b/tests/framework/web/FakePhp7Controller.php
index 4d267a85e15..381ba3e0ee3 100644
--- a/tests/framework/web/FakePhp7Controller.php
+++ b/tests/framework/web/FakePhp7Controller.php
@@ -17,11 +17,11 @@ class FakePhp7Controller extends Controller
 {
     public $enableCsrfValidation = false;
 
-    public function actionAksi1(int $foo, float $bar = null, bool $true, bool $false)
+    public function actionAksi1(int $foo, ?float $bar = null, bool $true, bool $false)
     {
     }
 
-    public function actionStringy(string $foo = null)
+    public function actionStringy(?string $foo = null)
     {
     }
 }