diff --git a/config/services.yaml b/config/services.yaml
index e4f668ccd..ef61daaca 100644
--- a/config/services.yaml
+++ b/config/services.yaml
@@ -15,9 +15,9 @@ services:
         autowire: true      # Automatically injects dependencies in your services.
         autoconfigure: true # Automatically registers your services as commands, event subscribers, etc.
         bind:               # defines the scalar arguments once and apply them to any service defined/created in this file
-            $locales: '%app_locales%'
-            $defaultLocale: '%locale%'
-            $emailSender: '%app.notifications.email_sender%'
+            string $locales: '%app_locales%'
+            string $defaultLocale: '%locale%'
+            string $emailSender: '%app.notifications.email_sender%'
 
     # makes classes in src/ available to be used as services
     # this creates a service per class whose id is the fully-qualified class name
diff --git a/src/Command/ListUsersCommand.php b/src/Command/ListUsersCommand.php
index 31703baf7..a1c8158c8 100644
--- a/src/Command/ListUsersCommand.php
+++ b/src/Command/ListUsersCommand.php
@@ -46,7 +46,7 @@ class ListUsersCommand extends Command
     private $emailSender;
     private $users;
 
-    public function __construct(MailerInterface $mailer, $emailSender, UserRepository $users)
+    public function __construct(MailerInterface $mailer, string $emailSender, UserRepository $users)
     {
         parent::__construct();
 
diff --git a/src/Entity/Post.php b/src/Entity/Post.php
index b9dd28403..479e481d8 100644
--- a/src/Entity/Post.php
+++ b/src/Entity/Post.php
@@ -93,7 +93,7 @@ class Post
     private $author;
 
     /**
-     * @var Comment[]|ArrayCollection
+     * @var Comment[]|Collection
      *
      * @ORM\OneToMany(
      *      targetEntity="Comment",
@@ -106,7 +106,7 @@ class Post
     private $comments;
 
     /**
-     * @var Tag[]|ArrayCollection
+     * @var Tag[]|Collection
      *
      * @ORM\ManyToMany(targetEntity="App\Entity\Tag", cascade={"persist"})
      * @ORM\JoinTable(name="symfony_demo_post_tag")
diff --git a/src/EventSubscriber/CommentNotificationSubscriber.php b/src/EventSubscriber/CommentNotificationSubscriber.php
index 35f194cd4..07675e746 100644
--- a/src/EventSubscriber/CommentNotificationSubscriber.php
+++ b/src/EventSubscriber/CommentNotificationSubscriber.php
@@ -31,7 +31,7 @@ class CommentNotificationSubscriber implements EventSubscriberInterface
     private $urlGenerator;
     private $sender;
 
-    public function __construct(MailerInterface $mailer, UrlGeneratorInterface $urlGenerator, TranslatorInterface $translator, $sender)
+    public function __construct(MailerInterface $mailer, UrlGeneratorInterface $urlGenerator, TranslatorInterface $translator, string $sender)
     {
         $this->mailer = $mailer;
         $this->urlGenerator = $urlGenerator;
diff --git a/src/Form/Type/DateTimePickerType.php b/src/Form/Type/DateTimePickerType.php
index 83a18802f..59aca6c30 100644
--- a/src/Form/Type/DateTimePickerType.php
+++ b/src/Form/Type/DateTimePickerType.php
@@ -61,7 +61,7 @@ public function configureOptions(OptionsResolver $resolver): void
     /**
      * {@inheritdoc}
      */
-    public function getParent()
+    public function getParent(): ?string
     {
         return DateTimeType::class;
     }
diff --git a/src/Form/Type/TagsInputType.php b/src/Form/Type/TagsInputType.php
index 8371a4cf5..5cc4b4ecd 100644
--- a/src/Form/Type/TagsInputType.php
+++ b/src/Form/Type/TagsInputType.php
@@ -63,7 +63,7 @@ public function buildView(FormView $view, FormInterface $form, array $options):
     /**
      * {@inheritdoc}
      */
-    public function getParent()
+    public function getParent(): ?string
     {
         return TextType::class;
     }
diff --git a/src/Security/PostVoter.php b/src/Security/PostVoter.php
index fbf7e6370..d4af08a8b 100644
--- a/src/Security/PostVoter.php
+++ b/src/Security/PostVoter.php
@@ -35,7 +35,7 @@ class PostVoter extends Voter
     /**
      * {@inheritdoc}
      */
-    protected function supports($attribute, $subject): bool
+    protected function supports(string $attribute, $subject): bool
     {
         // this voter is only executed for three specific permissions on Post objects
         return $subject instanceof Post && \in_array($attribute, [self::SHOW, self::EDIT, self::DELETE], true);
@@ -43,8 +43,10 @@ protected function supports($attribute, $subject): bool
 
     /**
      * {@inheritdoc}
+     *
+     * @param Post $post
      */
-    protected function voteOnAttribute($attribute, $post, TokenInterface $token): bool
+    protected function voteOnAttribute(string $attribute, $post, TokenInterface $token): bool
     {
         $user = $token->getUser();
 
diff --git a/src/Twig/SourceCodeExtension.php b/src/Twig/SourceCodeExtension.php
index f37a0ffde..0fc8bca59 100644
--- a/src/Twig/SourceCodeExtension.php
+++ b/src/Twig/SourceCodeExtension.php
@@ -46,6 +46,9 @@ public function getFunctions(): array
         ];
     }
 
+    /**
+     * @param string|TemplateWrapper|array $template
+     */
     public function showSourceCode(Environment $twig, $template): string
     {
         return $twig->render('debug/source_code.html.twig', [
@@ -94,10 +97,7 @@ private function getCallableReflector(callable $callable): \ReflectionFunctionAb
         return new \ReflectionFunction($callable);
     }
 
-    /**
-     * @param TemplateWrapper|Template $template
-     */
-    private function getTemplateSource($template): array
+    private function getTemplateSource(TemplateWrapper $template): array
     {
         $templateSource = $template->getSourceContext();