diff --git a/composer.json b/composer.json
index 47ad0df..07834d7 100644
--- a/composer.json
+++ b/composer.json
@@ -1,5 +1,8 @@
 {
 	"name": "drupal/simple_message",
 	"description": "Displays a simple message.",
-	"type": "drupal-module"
+	"type": "drupal-module",
+	"require": {
+		"php": "^7.4"
+	}
 }
diff --git a/src/DisplaySimpleMessage.php b/src/DisplaySimpleMessage.php
index 2e460b2..135410c 100644
--- a/src/DisplaySimpleMessage.php
+++ b/src/DisplaySimpleMessage.php
@@ -1,7 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Drupal\simple_message;
 
+use Drupal\Core\Config\Config;
 use Drupal\Core\Routing\AdminContext;
 use Drupal\Core\Messenger\MessengerInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
@@ -14,9 +17,9 @@ final class DisplaySimpleMessage implements EventSubscriberInterface {
 
   use StringTranslationTrait;
 
-  private $messenger;
-  private $adminContext;
-  private $config;
+  private MessengerInterface $messenger;
+  private AdminContext $adminContext;
+  private Config $config;
 
   public function __construct(
     MessengerInterface $messenger,
@@ -28,7 +31,7 @@ final class DisplaySimpleMessage implements EventSubscriberInterface {
     $this->config = $configFactory->get('simple_message.config');
   }
 
-  public function displayMessage(GetResponseEvent $event) {
+  public function displayMessage(GetResponseEvent $event): void {
     if ($this->adminContext->isAdminRoute()) {
       return;
     }