diff --git a/phpstan.neon b/phpstan.neon
index 8aea6a8e..a335376b 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -1,5 +1,6 @@
 parameters:
     inferPrivatePropertyTypeFromConstructor: true
+    treatPhpDocTypesAsCertain: false
 
     symfony:
         container_xml_path: '%rootDir%/../../../var/cache/dev/srcApp_KernelDevDebugContainer.xml'
diff --git a/src/Command/CleanAttachmentsCommand.php b/src/Command/CleanAttachmentsCommand.php
index f099baf2..c38c5dfe 100644
--- a/src/Command/CleanAttachmentsCommand.php
+++ b/src/Command/CleanAttachmentsCommand.php
@@ -86,7 +86,7 @@ class CleanAttachmentsCommand extends Command
 
         $table = new Table($output);
         $table->setHeaders(['Filename', 'MIME Type', 'Last modified date']);
-        $dateformatter = IntlDateFormatter::create(null, IntlDateFormatter::SHORT, IntlDateFormatter::SHORT);
+        $dateformatter = IntlDateFormatter::create(\Locale::getDefault(), IntlDateFormatter::SHORT, IntlDateFormatter::SHORT);
 
         foreach ($finder as $file) {
             //If not attachment object uses this file, print it
diff --git a/src/Command/SetPasswordCommand.php b/src/Command/SetPasswordCommand.php
index ac895d77..a95819af 100644
--- a/src/Command/SetPasswordCommand.php
+++ b/src/Command/SetPasswordCommand.php
@@ -64,14 +64,15 @@ class SetPasswordCommand extends Command
 
         /** @var User[] $users */
         $users = $this->entityManager->getRepository(User::class)->findBy(['name' => $user_name]);
-        $user = $users[0];
 
-        if (null === $user) {
+        if (empty($users)) {
             $io->error(sprintf('No user with the given username %s found in the database!', $user_name));
 
             return 1;
         }
 
+        $user = $users[0];
+
         $io->note('User found!');
 
         $proceed = $io->confirm(
diff --git a/src/Controller/AdminPages/BaseAdminController.php b/src/Controller/AdminPages/BaseAdminController.php
index b5f5ed31..3a76c15d 100644
--- a/src/Controller/AdminPages/BaseAdminController.php
+++ b/src/Controller/AdminPages/BaseAdminController.php
@@ -130,7 +130,7 @@ abstract class BaseAdminController extends AbstractController
 
     protected function _new(Request $request, EntityManagerInterface $em, EntityImporter $importer)
     {
-        /** @var StructuralDBElement $new_entity */
+        /** @var StructuralDBElement|User $new_entity */
         $new_entity = new $this->entity_class();
 
         $this->denyAccessUnlessGranted('read', $new_entity);
diff --git a/src/Controller/UserSettingsController.php b/src/Controller/UserSettingsController.php
index 1338d02f..a2911bbf 100644
--- a/src/Controller/UserSettingsController.php
+++ b/src/Controller/UserSettingsController.php
@@ -236,9 +236,9 @@ class UserSettingsController extends AbstractController
                     ],
                 ],
                 'constraints' => [new Length([
-                    'min' => 6,
-                    'max' => 128,
-                ])],
+                                                 'min' => 6,
+                                                 'max' => 128,
+                                             ])],
             ])
             ->add('submit', SubmitType::class, ['label' => 'save'])
             ->getForm();
@@ -278,15 +278,13 @@ class UserSettingsController extends AbstractController
                 return $this->redirectToRoute('user_settings');
             }
 
-            if ($google_enabled) {
-                //Remove secret to disable google authenticator
-                $user->setGoogleAuthenticatorSecret(null);
-                $backupCodeManager->disableBackupCodesIfUnused($user);
-                $em->flush();
-                $this->addFlash('success', 'user.settings.2fa.google.disabled');
+            //Remove secret to disable google authenticator
+            $user->setGoogleAuthenticatorSecret(null);
+            $backupCodeManager->disableBackupCodesIfUnused($user);
+            $em->flush();
+            $this->addFlash('success', 'user.settings.2fa.google.disabled');
 
-                return $this->redirectToRoute('user_settings');
-            }
+            return $this->redirectToRoute('user_settings');
         }
 
         $backup_form = $this->get('form.factory')->createNamedBuilder('backup_codes')->add('reset_codes', SubmitType::class, [
diff --git a/src/DataTables/Column/EntityColumn.php b/src/DataTables/Column/EntityColumn.php
index b8994e5f..f4ce6c35 100644
--- a/src/DataTables/Column/EntityColumn.php
+++ b/src/DataTables/Column/EntityColumn.php
@@ -57,7 +57,7 @@ class EntityColumn extends AbstractColumn
         return $value;
     }
 
-    public function configureOptions(OptionsResolver $resolver): void
+    public function configureOptions(OptionsResolver $resolver)
     {
         parent::configureOptions($resolver);
 
@@ -69,7 +69,7 @@ class EntityColumn extends AbstractColumn
 
         $resolver->setDefault('render', function (Options $options) {
             return function ($value, Part $context) use ($options) {
-                /** @var DBElement $entity */
+                /** @var DBElement|null $entity */
                 $entity = $this->accessor->getValue($context, $options['property']);
 
                 if ($entity) {
@@ -83,7 +83,10 @@ class EntityColumn extends AbstractColumn
 
                     return sprintf('<i>%s</i>', $value);
                 }
+                throw new \InvalidArgumentException('$entity must not be null!');
             };
         });
+
+        return $this;
     }
 }
diff --git a/src/DataTables/Column/LogEntryTargetColumn.php b/src/DataTables/Column/LogEntryTargetColumn.php
index 327b7f07..db937d77 100644
--- a/src/DataTables/Column/LogEntryTargetColumn.php
+++ b/src/DataTables/Column/LogEntryTargetColumn.php
@@ -58,9 +58,10 @@ class LogEntryTargetColumn extends AbstractColumn
         return $value;
     }
 
-    public function configureOptions(OptionsResolver $resolver): void
+    public function configureOptions(OptionsResolver $resolver)
     {
         parent::configureOptions($resolver);
+        return $this;
     }
 
     public function render($value, $context)
diff --git a/src/DataTables/Column/PartAttachmentsColumn.php b/src/DataTables/Column/PartAttachmentsColumn.php
index a4530741..974aa1d7 100644
--- a/src/DataTables/Column/PartAttachmentsColumn.php
+++ b/src/DataTables/Column/PartAttachmentsColumn.php
@@ -91,8 +91,9 @@ class PartAttachmentsColumn extends AbstractColumn
         return $tmp;
     }
 
-    public function configureOptions(OptionsResolver $resolver): void
+    public function configureOptions(OptionsResolver $resolver)
     {
         parent::configureOptions($resolver);
+        return $this;
     }
 }
diff --git a/src/Entity/Attachments/Attachment.php b/src/Entity/Attachments/Attachment.php
index 0288c416..b9a507a6 100644
--- a/src/Entity/Attachments/Attachment.php
+++ b/src/Entity/Attachments/Attachment.php
@@ -79,7 +79,7 @@ abstract class Attachment extends NamedDBElement
     public const ALLOWED_ELEMENT_CLASS = '';
 
     /**
-     * @var string the original filename the file had, when the user uploaded it
+     * @var string|null the original filename the file had, when the user uploaded it
      * @ORM\Column(type="string", nullable=true)
      */
     protected $original_filename;
diff --git a/src/Entity/Attachments/AttachmentContainingDBElement.php b/src/Entity/Attachments/AttachmentContainingDBElement.php
index cc447695..7fc3c153 100644
--- a/src/Entity/Attachments/AttachmentContainingDBElement.php
+++ b/src/Entity/Attachments/AttachmentContainingDBElement.php
@@ -43,7 +43,7 @@ abstract class AttachmentContainingDBElement extends NamedDBElement
      *
      * Mapping is done in sub classes like part
      */
-    protected $attachments = [];
+    protected $attachments;
 
     public function __construct()
     {
diff --git a/src/Entity/Base/DBElement.php b/src/Entity/Base/DBElement.php
index 8d260e34..4a0cf09c 100644
--- a/src/Entity/Base/DBElement.php
+++ b/src/Entity/Base/DBElement.php
@@ -58,7 +58,8 @@ use Symfony\Component\Serializer\Annotation\Groups;
  */
 abstract class DBElement
 {
-    /** @var int The Identification number for this part. This value is unique for the element in this table.
+    /** @var int|null The Identification number for this part. This value is unique for the element in this table.
+     * Null if the element is not saved to DB yet.
      * @ORM\Column(type="integer")
      * @ORM\Id()
      * @ORM\GeneratedValue()
diff --git a/src/Entity/Base/StructuralDBElement.php b/src/Entity/Base/StructuralDBElement.php
index 53d14b58..b141d524 100644
--- a/src/Entity/Base/StructuralDBElement.php
+++ b/src/Entity/Base/StructuralDBElement.php
@@ -81,10 +81,11 @@ abstract class StructuralDBElement extends AttachmentContainingDBElement
      * We can not define the mapping here or we will get an exception. Unfortunately we have to do the mapping in the
      * subclasses.
      *
-     * @var StructuralDBElement[]
+     * @var StructuralDBElement[]|Collection
      * @Groups({"include_children"})
      */
-    protected $children = [];
+    protected $children;
+
     /**
      * @var StructuralDBElement
      * @NoneOfItsChildren()
@@ -249,7 +250,7 @@ abstract class StructuralDBElement extends AttachmentContainingDBElement
     /**
      * Get all sub elements of this element.
      *
-     * @return Collection<static> all subelements as an array of objects (sorted by their full path)
+     * @return Collection<static>|iterable all subelements as an array of objects (sorted by their full path)
      */
     public function getSubelements(): iterable
     {
@@ -257,7 +258,7 @@ abstract class StructuralDBElement extends AttachmentContainingDBElement
     }
 
     /**
-     * @return Collection<static>
+     * @return Collection<static>|iterable
      */
     public function getChildren(): iterable
     {
@@ -311,9 +312,17 @@ abstract class StructuralDBElement extends AttachmentContainingDBElement
         return $this;
     }
 
-    public function setChildren(array $element): self
+    /**
+     * @param  static[]|Collection  $elements
+     * @return $this
+     */
+    public function setChildren($elements): self
     {
-        $this->children = $element;
+        if (!is_array($elements) && !$elements instanceof Collection) {
+           throw new InvalidArgumentException('$elements must be an array or Collection!');
+        }
+
+        $this->children = $elements;
 
         return $this;
     }
diff --git a/src/Entity/Parts/Supplier.php b/src/Entity/Parts/Supplier.php
index 9c8da03f..0940755f 100644
--- a/src/Entity/Parts/Supplier.php
+++ b/src/Entity/Parts/Supplier.php
@@ -93,7 +93,7 @@ class Supplier extends Company
     protected $default_currency;
 
     /**
-     * @var float|null the shipping costs that have to be paid, when ordering via this supplier
+     * @var string|null the shipping costs that have to be paid, when ordering via this supplier
      * @ORM\Column(name="shipping_costs", nullable=true, type="decimal", precision=11, scale=5)
      * @Assert\PositiveOrZero()
      */
diff --git a/src/Entity/PriceInformations/Pricedetail.php b/src/Entity/PriceInformations/Pricedetail.php
index 133c5acb..22ecac1d 100644
--- a/src/Entity/PriceInformations/Pricedetail.php
+++ b/src/Entity/PriceInformations/Pricedetail.php
@@ -109,7 +109,7 @@ class Pricedetail extends DBElement
     protected $manual_input = true;
 
     /**
-     * @var Orderdetail
+     * @var Orderdetail|null
      * @ORM\ManyToOne(targetEntity="Orderdetail", inversedBy="pricedetails")
      * @ORM\JoinColumn(name="orderdetails_id", referencedColumnName="id", nullable=false, onDelete="CASCADE")
      * @Assert\NotNull()
diff --git a/src/Entity/UserSystem/U2FKey.php b/src/Entity/UserSystem/U2FKey.php
index 446692f5..c77af17c 100644
--- a/src/Entity/UserSystem/U2FKey.php
+++ b/src/Entity/UserSystem/U2FKey.php
@@ -103,9 +103,10 @@ class U2FKey implements TwoFactorKeyInterface
         return $this->keyHandle;
     }
 
-    public function setKeyHandle($keyHandle): void
+    public function setKeyHandle($keyHandle): self
     {
         $this->keyHandle = $keyHandle;
+        return $this;
     }
 
     public function getPublicKey()
@@ -113,9 +114,10 @@ class U2FKey implements TwoFactorKeyInterface
         return $this->publicKey;
     }
 
-    public function setPublicKey($publicKey): void
+    public function setPublicKey($publicKey): self
     {
         $this->publicKey = $publicKey;
+        return $this;
     }
 
     public function getCertificate()
@@ -123,9 +125,10 @@ class U2FKey implements TwoFactorKeyInterface
         return $this->certificate;
     }
 
-    public function setCertificate($certificate): void
+    public function setCertificate($certificate): self
     {
         $this->certificate = $certificate;
+        return $this;
     }
 
     public function getCounter()
@@ -133,9 +136,10 @@ class U2FKey implements TwoFactorKeyInterface
         return $this->counter;
     }
 
-    public function setCounter($counter): void
+    public function setCounter($counter): self
     {
         $this->counter = $counter;
+        return $this;
     }
 
     public function getName()
@@ -143,9 +147,10 @@ class U2FKey implements TwoFactorKeyInterface
         return $this->name;
     }
 
-    public function setName($name): void
+    public function setName($name): self
     {
         $this->name = $name;
+        return $this;
     }
 
     /**
@@ -173,7 +178,7 @@ class U2FKey implements TwoFactorKeyInterface
      *
      * @return $this
      */
-    public function setUser(TwoFactorInterface $new_user): self
+    public function setUser(User $new_user): self
     {
         $this->user = $new_user;
 
diff --git a/src/Entity/UserSystem/User.php b/src/Entity/UserSystem/User.php
index 40dc02bf..3cdfe9e1 100644
--- a/src/Entity/UserSystem/User.php
+++ b/src/Entity/UserSystem/User.php
@@ -234,7 +234,7 @@ class User extends AttachmentContainingDBElement implements UserInterface, HasPe
      */
     protected $attachments;
 
-    /** @var DateTime The time when the backup codes were generated
+    /** @var DateTime|null The time when the backup codes were generated
      * @ORM\Column(type="datetime", nullable=true)
      */
     protected $backupCodesGenerationDate;
@@ -351,9 +351,9 @@ class User extends AttachmentContainingDBElement implements UserInterface, HasPe
     /**
      * @see UserInterface
      */
-    public function getSalt(): void
+    public function getSalt(): ?string
     {
-        // not needed when using the "bcrypt" algorithm in security.yaml
+        return null;
     }
 
     /**
diff --git a/src/EventSubscriber/SymfonyDebugToolbarSubscriber.php b/src/EventSubscriber/SymfonyDebugToolbarSubscriber.php
index c42ad500..68c3ca12 100644
--- a/src/EventSubscriber/SymfonyDebugToolbarSubscriber.php
+++ b/src/EventSubscriber/SymfonyDebugToolbarSubscriber.php
@@ -67,6 +67,6 @@ final class SymfonyDebugToolbarSubscriber implements EventSubscriberInterface
         }
 
         $response = $event->getResponse();
-        $response->headers->set('Symfony-Debug-Toolbar-Replace', 1);
+        $response->headers->set('Symfony-Debug-Toolbar-Replace', '1');
     }
 }
diff --git a/src/EventSubscriber/U2FRegistrationSubscriber.php b/src/EventSubscriber/U2FRegistrationSubscriber.php
index 54622d6c..3e55d03a 100644
--- a/src/EventSubscriber/U2FRegistrationSubscriber.php
+++ b/src/EventSubscriber/U2FRegistrationSubscriber.php
@@ -25,6 +25,7 @@ declare(strict_types=1);
 namespace App\EventSubscriber;
 
 use App\Entity\UserSystem\U2FKey;
+use App\Entity\UserSystem\User;
 use Doctrine\ORM\EntityManagerInterface;
 use R\U2FTwoFactorBundle\Event\RegisterEvent;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
@@ -63,6 +64,10 @@ final class U2FRegistrationSubscriber implements EventSubscriberInterface
         //Skip adding of U2F key on demo mode
         if (! $this->demo_mode) {
             $user = $event->getUser();
+            if (!$user instanceof User) {
+                throw new \InvalidArgumentException("Only User objects can be registered for U2F!");
+            }
+
             $registration = $event->getRegistration();
             $newKey = new U2FKey();
             $newKey->fromRegistrationData($registration);
diff --git a/src/Helpers/Trees/StructuralDBElementIterator.php b/src/Helpers/Trees/StructuralDBElementIterator.php
index 285213b0..fe0ab0d4 100644
--- a/src/Helpers/Trees/StructuralDBElementIterator.php
+++ b/src/Helpers/Trees/StructuralDBElementIterator.php
@@ -26,6 +26,7 @@ namespace App\Helpers\Trees;
 
 use App\Entity\Base\StructuralDBElement;
 use ArrayIterator;
+use Doctrine\Common\Collections\Collection;
 use RecursiveIterator;
 
 final class StructuralDBElementIterator extends ArrayIterator implements RecursiveIterator
@@ -48,6 +49,15 @@ final class StructuralDBElementIterator extends ArrayIterator implements Recursi
         /** @var StructuralDBElement $element */
         $element = $this->current();
 
-        return new self($element->getSubelements()->toArray());
+        $subelements = $element->getSubelements();
+        if (is_array($subelements)) {
+            $array = $subelements;
+        } elseif ($subelements instanceof Collection) {
+            $array = $subelements->toArray();
+        } else {
+            throw new \InvalidArgumentException('Invalid subelements type on $element!');
+        }
+
+        return new self($array);
     }
 }
diff --git a/src/Services/Attachments/AttachmentReverseSearch.php b/src/Services/Attachments/AttachmentReverseSearch.php
index ab3a7d18..10a08773 100644
--- a/src/Services/Attachments/AttachmentReverseSearch.php
+++ b/src/Services/Attachments/AttachmentReverseSearch.php
@@ -91,7 +91,7 @@ class AttachmentReverseSearch
         $this->cacheManager->remove($this->attachmentURLGenerator->absolutePathToAssetPath($file->getPathname()));
 
         $fs = new Filesystem();
-        $fs->remove($file);
+        $fs->remove($file->getPathname());
 
         return true;
     }
diff --git a/src/Services/EntityImporter.php b/src/Services/EntityImporter.php
index 5c6a4955..7cfb7a7a 100644
--- a/src/Services/EntityImporter.php
+++ b/src/Services/EntityImporter.php
@@ -25,6 +25,7 @@ declare(strict_types=1);
 namespace App\Services;
 
 use App\Entity\Base\StructuralDBElement;
+use Symfony\Bundle\MakerBundle\Str;
 use function count;
 use Doctrine\ORM\EntityManagerInterface;
 use InvalidArgumentException;
@@ -133,7 +134,7 @@ class EntityImporter
             $tmp = $this->validator->validate($entity);
 
             //When no validation error occured, persist entity to database (cascade must be set in entity)
-            if (0 === count($errors)) {
+            if (empty($tmp)) {
                 $this->em->persist($entity);
             } else { //Log validation errors to global log.
                 $errors[$entity->getFullPath()] = $tmp;
@@ -210,7 +211,7 @@ class EntityImporter
      * This functions corrects the parent setting based on the children value of the parent.
      *
      * @param iterable $entities the list of entities that should be fixed
-     * @param null     $parent   the parent, to which the entity should be set
+     * @param null|StructuralDBElement $parent   the parent, to which the entity should be set
      */
     protected function correctParentEntites(iterable $entities, $parent = null): void
     {
diff --git a/src/Services/PasswordResetManager.php b/src/Services/PasswordResetManager.php
index 7657aa31..e83279c8 100644
--- a/src/Services/PasswordResetManager.php
+++ b/src/Services/PasswordResetManager.php
@@ -96,19 +96,19 @@ class PasswordResetManager
     /**
      * Sets the new password of the user with the given name, if the token is valid.
      *
-     * @param string $user         The name of the user, which password should be reset
+     * @param string $username     The name of the user, which password should be reset
      * @param string $token        The token that should be used to reset the password
      * @param string $new_password The new password that should be applied to user
      *
      * @return bool Returns true, if the new password was applied. False, if either the username is unknown or the
      *              token is invalid or expired.
      */
-    public function setNewPassword(string $user, string $token, string $new_password): bool
+    public function setNewPassword(string $username, string $token, string $new_password): bool
     {
         //Try to find the user
         $repo = $this->em->getRepository(User::class);
-        /** @var User $user */
-        $user = $repo->findOneBy(['name' => $user]);
+        /** @var User|null $user */
+        $user = $repo->findOneBy(['name' => $username]);
 
         //If no user matching the name, show an error message
         if (null === $user) {
diff --git a/src/Services/PermissionResolver.php b/src/Services/PermissionResolver.php
index eb4dc78f..2b6a29c1 100644
--- a/src/Services/PermissionResolver.php
+++ b/src/Services/PermissionResolver.php
@@ -25,6 +25,7 @@ declare(strict_types=1);
 namespace App\Services;
 
 use App\Configuration\PermissionsConfiguration;
+use App\Entity\UserSystem\Group;
 use App\Entity\UserSystem\User;
 use App\Security\Interfaces\HasPermissionsInterface;
 use Symfony\Component\Config\ConfigCache;
@@ -109,6 +110,7 @@ class PermissionResolver
             return $allowed;
         }
 
+        /** @var HasPermissionsInterface $parent */
         $parent = $user->getGroup();
         while (null !== $parent) { //The top group, has parent == null
             //Check if our current element gives a info about disallow/allow
diff --git a/src/Services/PricedetailHelper.php b/src/Services/PricedetailHelper.php
index 6039549c..81c1bcca 100644
--- a/src/Services/PricedetailHelper.php
+++ b/src/Services/PricedetailHelper.php
@@ -191,7 +191,7 @@ class PricedetailHelper
         $val_base = $value;
         if (null !== $originCurrency) {
             //Without an exchange rate we can not calculate the exchange rate
-            if (0 === (float) $originCurrency->getExchangeRate()) {
+            if (0.0 === (float) $originCurrency->getExchangeRate()) {
                 return null;
             }