diff --git a/module/Common/src/Entity/AbstractEntity.php b/module/Common/src/Entity/AbstractEntity.php
index 8b71b4ec..b798a087 100644
--- a/module/Common/src/Entity/AbstractEntity.php
+++ b/module/Common/src/Entity/AbstractEntity.php
@@ -8,26 +8,19 @@ use Doctrine\ORM\Mapping as ORM;
 abstract class AbstractEntity
 {
     /**
-     * @var int
+     * @var string
      * @ORM\Id
      * @ORM\GeneratedValue(strategy="IDENTITY")
      * @ORM\Column(name="id", type="bigint", options={"unsigned"=true})
      */
     protected $id;
 
-    /**
-     * @return int
-     */
-    public function getId()
+    public function getId(): string
     {
         return $this->id;
     }
 
-    /**
-     * @param int $id
-     * @return $this
-     */
-    public function setId($id)
+    public function setId(string $id): self
     {
         $this->id = $id;
         return $this;
diff --git a/module/Core/src/Entity/ShortUrl.php b/module/Core/src/Entity/ShortUrl.php
index def81f01..48f597f3 100644
--- a/module/Core/src/Entity/ShortUrl.php
+++ b/module/Core/src/Entity/ShortUrl.php
@@ -7,13 +7,14 @@ use Doctrine\Common\Collections\ArrayCollection;
 use Doctrine\Common\Collections\Collection;
 use Doctrine\ORM\Mapping as ORM;
 use Shlinkio\Shlink\Common\Entity\AbstractEntity;
+use Shlinkio\Shlink\Core\Repository\ShortUrlRepository;
 
 /**
  * Class ShortUrl
  * @author
  * @link
  *
- * @ORM\Entity(repositoryClass="Shlinkio\Shlink\Core\Repository\ShortUrlRepository")
+ * @ORM\Entity(repositoryClass=ShortUrlRepository::class)
  * @ORM\Table(name="short_urls")
  */
 class ShortUrl extends AbstractEntity
@@ -22,7 +23,7 @@ class ShortUrl extends AbstractEntity
      * @var string
      * @ORM\Column(name="original_url", type="string", nullable=false, length=1024)
      */
-    protected $originalUrl;
+    private $originalUrl;
     /**
      * @var string
      * @ORM\Column(
@@ -33,17 +34,17 @@ class ShortUrl extends AbstractEntity
      *     unique=true
      * )
      */
-    protected $shortCode;
+    private $shortCode;
     /**
      * @var \DateTime
      * @ORM\Column(name="date_created", type="datetime")
      */
-    protected $dateCreated;
+    private $dateCreated;
     /**
      * @var Collection|Visit[]
      * @ORM\OneToMany(targetEntity=Visit::class, mappedBy="shortUrl", fetch="EXTRA_LAZY")
      */
-    protected $visits;
+    private $visits;
     /**
      * @var Collection|Tag[]
      * @ORM\ManyToMany(targetEntity=Tag::class, cascade={"persist"})
@@ -53,46 +54,36 @@ class ShortUrl extends AbstractEntity
      *     @ORM\JoinColumn(name="tag_id", referencedColumnName="id")
      * })
      */
-    protected $tags;
+    private $tags;
     /**
      * @var \DateTime
      * @ORM\Column(name="valid_since", type="datetime", nullable=true)
      */
-    protected $validSince;
+    private $validSince;
     /**
      * @var \DateTime
      * @ORM\Column(name="valid_until", type="datetime", nullable=true)
      */
-    protected $validUntil;
+    private $validUntil;
     /**
      * @var integer
      * @ORM\Column(name="max_visits", type="integer", nullable=true)
      */
-    protected $maxVisits;
+    private $maxVisits;
 
-    /**
-     * ShortUrl constructor.
-     */
     public function __construct()
     {
+        $this->shortCode = '';
         $this->dateCreated = new \DateTime();
         $this->visits = new ArrayCollection();
-        $this->shortCode = '';
         $this->tags = new ArrayCollection();
     }
 
-    /**
-     * @return string
-     */
     public function getLongUrl(): string
     {
         return $this->originalUrl;
     }
 
-    /**
-     * @param string $longUrl
-     * @return $this
-     */
     public function setLongUrl(string $longUrl): self
     {
         $this->originalUrl = $longUrl;
@@ -100,7 +91,6 @@ class ShortUrl extends AbstractEntity
     }
 
     /**
-     * @return string
      * @deprecated Use getLongUrl() instead
      */
     public function getOriginalUrl(): string
@@ -109,8 +99,6 @@ class ShortUrl extends AbstractEntity
     }
 
     /**
-     * @param string $originalUrl
-     * @return $this
      * @deprecated Use setLongUrl() instead
      */
     public function setOriginalUrl(string $originalUrl): self
@@ -118,37 +106,23 @@ class ShortUrl extends AbstractEntity
         return $this->setLongUrl($originalUrl);
     }
 
-    /**
-     * @return string
-     */
     public function getShortCode(): string
     {
         return $this->shortCode;
     }
 
-    /**
-     * @param string $shortCode
-     * @return $this
-     */
-    public function setShortCode(string $shortCode)
+    public function setShortCode(string $shortCode): self
     {
         $this->shortCode = $shortCode;
         return $this;
     }
 
-    /**
-     * @return \DateTime
-     */
     public function getDateCreated(): \DateTime
     {
         return $this->dateCreated;
     }
 
-    /**
-     * @param \DateTime $dateCreated
-     * @return $this
-     */
-    public function setDateCreated(\DateTime $dateCreated)
+    public function setDateCreated(\DateTime $dateCreated): self
     {
         $this->dateCreated = $dateCreated;
         return $this;
@@ -164,55 +138,36 @@ class ShortUrl extends AbstractEntity
 
     /**
      * @param Collection|Tag[] $tags
-     * @return $this
      */
-    public function setTags(Collection $tags)
+    public function setTags(Collection $tags): self
     {
         $this->tags = $tags;
         return $this;
     }
 
-    /**
-     * @param Tag $tag
-     * @return $this
-     */
-    public function addTag(Tag $tag)
+    public function addTag(Tag $tag): self
     {
         $this->tags->add($tag);
         return $this;
     }
 
-    /**
-     * @return \DateTime|null
-     */
-    public function getValidSince()
+    public function getValidSince(): ?\DateTime
     {
         return $this->validSince;
     }
 
-    /**
-     * @param \DateTime|null $validSince
-     * @return $this|self
-     */
-    public function setValidSince($validSince): self
+    public function setValidSince(?\DateTime $validSince): self
     {
         $this->validSince = $validSince;
         return $this;
     }
 
-    /**
-     * @return \DateTime|null
-     */
-    public function getValidUntil()
+    public function getValidUntil(): ?\DateTime
     {
         return $this->validUntil;
     }
 
-    /**
-     * @param \DateTime|null $validUntil
-     * @return $this|self
-     */
-    public function setValidUntil($validUntil): self
+    public function setValidUntil(?\DateTime $validUntil): self
     {
         $this->validUntil = $validUntil;
         return $this;
@@ -224,7 +179,7 @@ class ShortUrl extends AbstractEntity
     }
 
     /**
-     * @param Collection $visits
+     * @param Collection|Visit[] $visits
      * @return ShortUrl
      * @internal
      */
@@ -234,19 +189,12 @@ class ShortUrl extends AbstractEntity
         return $this;
     }
 
-    /**
-     * @return int|null
-     */
-    public function getMaxVisits()
+    public function getMaxVisits(): ?int
     {
         return $this->maxVisits;
     }
 
-    /**
-     * @param int|null $maxVisits
-     * @return $this|self
-     */
-    public function setMaxVisits($maxVisits): self
+    public function setMaxVisits(?int $maxVisits): self
     {
         $this->maxVisits = $maxVisits;
         return $this;
diff --git a/module/Core/src/Entity/Tag.php b/module/Core/src/Entity/Tag.php
index cd895268..aad6b713 100644
--- a/module/Core/src/Entity/Tag.php
+++ b/module/Core/src/Entity/Tag.php
@@ -21,39 +21,25 @@ class Tag extends AbstractEntity implements \JsonSerializable
      * @var string
      * @ORM\Column(unique=true)
      */
-    protected $name;
+    private $name;
 
     public function __construct($name = null)
     {
         $this->name = $name;
     }
 
-    /**
-     * @return string
-     */
-    public function getName()
+    public function getName(): string
     {
         return $this->name;
     }
 
-    /**
-     * @param string $name
-     * @return $this
-     */
-    public function setName($name)
+    public function setName(string $name)
     {
         $this->name = $name;
         return $this;
     }
 
-    /**
-     * Specify data which should be serialized to JSON
-     * @link http://php.net/manual/en/jsonserializable.jsonserialize.php
-     * @return mixed data which can be serialized by <b>json_encode</b>,
-     * which is a value of any type other than a resource.
-     * @since 5.4.0
-     */
-    public function jsonSerialize()
+    public function jsonSerialize(): string
     {
         return $this->name;
     }
diff --git a/module/Core/src/Entity/Visit.php b/module/Core/src/Entity/Visit.php
index 9390005c..f23657d4 100644
--- a/module/Core/src/Entity/Visit.php
+++ b/module/Core/src/Entity/Visit.php
@@ -7,13 +7,14 @@ use Doctrine\ORM\Mapping as ORM;
 use Shlinkio\Shlink\Common\Entity\AbstractEntity;
 use Shlinkio\Shlink\Common\Exception\WrongIpException;
 use Shlinkio\Shlink\Common\Util\IpAddress;
+use Shlinkio\Shlink\Core\Repository\VisitRepository;
 
 /**
  * Class Visit
  * @author
  * @link
  *
- * @ORM\Entity(repositoryClass="Shlinkio\Shlink\Core\Repository\VisitRepository")
+ * @ORM\Entity(repositoryClass=VisitRepository::class)
  * @ORM\Table(name="visits")
  */
 class Visit extends AbstractEntity implements \JsonSerializable
diff --git a/module/Core/src/Entity/VisitLocation.php b/module/Core/src/Entity/VisitLocation.php
index ca30596a..2027f7e3 100644
--- a/module/Core/src/Entity/VisitLocation.php
+++ b/module/Core/src/Entity/VisitLocation.php
@@ -21,159 +21,110 @@ class VisitLocation extends AbstractEntity implements ArraySerializableInterface
      * @var string
      * @ORM\Column(nullable=true)
      */
-    protected $countryCode;
+    private $countryCode;
     /**
      * @var string
      * @ORM\Column(nullable=true)
      */
-    protected $countryName;
+    private $countryName;
     /**
      * @var string
      * @ORM\Column(nullable=true)
      */
-    protected $regionName;
+    private $regionName;
     /**
      * @var string
      * @ORM\Column(nullable=true)
      */
-    protected $cityName;
+    private $cityName;
     /**
      * @var string
      * @ORM\Column(nullable=true)
      */
-    protected $latitude;
+    private $latitude;
     /**
      * @var string
      * @ORM\Column(nullable=true)
      */
-    protected $longitude;
+    private $longitude;
     /**
      * @var string
      * @ORM\Column(nullable=true)
      */
-    protected $timezone;
+    private $timezone;
 
-    /**
-     * @return string
-     */
-    public function getCountryCode()
+    public function getCountryCode(): string
     {
-        return $this->countryCode;
+        return $this->countryCode ?? '';
     }
 
-    /**
-     * @param string $countryCode
-     * @return $this
-     */
-    public function setCountryCode($countryCode)
+    public function setCountryCode(string $countryCode)
     {
         $this->countryCode = $countryCode;
         return $this;
     }
 
-    /**
-     * @return string
-     */
-    public function getCountryName()
+    public function getCountryName(): string
     {
-        return $this->countryName;
+        return $this->countryName ?? '';
     }
 
-    /**
-     * @param string $countryName
-     * @return $this
-     */
-    public function setCountryName($countryName)
+    public function setCountryName(string $countryName): self
     {
         $this->countryName = $countryName;
         return $this;
     }
 
-    /**
-     * @return string
-     */
-    public function getRegionName()
+    public function getRegionName(): string
     {
-        return $this->regionName;
+        return $this->regionName ?? '';
     }
 
-    /**
-     * @param string $regionName
-     * @return $this
-     */
-    public function setRegionName($regionName)
+    public function setRegionName(string $regionName): self
     {
         $this->regionName = $regionName;
         return $this;
     }
 
-    /**
-     * @return string
-     */
-    public function getCityName()
+    public function getCityName(): string
     {
-        return $this->cityName;
+        return $this->cityName ?? '';
     }
 
-    /**
-     * @param string $cityName
-     * @return $this
-     */
-    public function setCityName($cityName)
+    public function setCityName(string $cityName): self
     {
         $this->cityName = $cityName;
         return $this;
     }
 
-    /**
-     * @return string
-     */
-    public function getLatitude()
+    public function getLatitude(): string
     {
-        return $this->latitude;
+        return $this->latitude ?? '';
     }
 
-    /**
-     * @param string $latitude
-     * @return $this
-     */
-    public function setLatitude($latitude)
+    public function setLatitude(string $latitude): self
     {
         $this->latitude = $latitude;
         return $this;
     }
 
-    /**
-     * @return string
-     */
-    public function getLongitude()
+    public function getLongitude(): string
     {
-        return $this->longitude;
+        return $this->longitude ?? '';
     }
 
-    /**
-     * @param string $longitude
-     * @return $this
-     */
-    public function setLongitude($longitude)
+    public function setLongitude(string $longitude): self
     {
         $this->longitude = $longitude;
         return $this;
     }
 
-    /**
-     * @return string
-     */
-    public function getTimezone()
+    public function getTimezone(): string
     {
-        return $this->timezone;
+        return $this->timezone ?? '';
     }
 
-    /**
-     * @param string $timezone
-     * @return $this
-     */
-    public function setTimezone($timezone)
+    public function setTimezone(string $timezone): self
     {
         $this->timezone = $timezone;
         return $this;
@@ -181,41 +132,36 @@ class VisitLocation extends AbstractEntity implements ArraySerializableInterface
 
     /**
      * Exchange internal values from provided array
-     *
-     * @param  array $array
-     * @return void
      */
-    public function exchangeArray(array $array)
+    public function exchangeArray(array $array): void
     {
-        if (array_key_exists('country_code', $array)) {
+        if (\array_key_exists('country_code', $array)) {
             $this->setCountryCode($array['country_code']);
         }
-        if (array_key_exists('country_name', $array)) {
+        if (\array_key_exists('country_name', $array)) {
             $this->setCountryName($array['country_name']);
         }
-        if (array_key_exists('region_name', $array)) {
+        if (\array_key_exists('region_name', $array)) {
             $this->setRegionName($array['region_name']);
         }
-        if (array_key_exists('city', $array)) {
+        if (\array_key_exists('city', $array)) {
             $this->setCityName($array['city']);
         }
-        if (array_key_exists('latitude', $array)) {
+        if (\array_key_exists('latitude', $array)) {
             $this->setLatitude($array['latitude']);
         }
-        if (array_key_exists('longitude', $array)) {
+        if (\array_key_exists('longitude', $array)) {
             $this->setLongitude($array['longitude']);
         }
-        if (array_key_exists('time_zone', $array)) {
+        if (\array_key_exists('time_zone', $array)) {
             $this->setTimezone($array['time_zone']);
         }
     }
 
     /**
      * Return an array representation of the object
-     *
-     * @return array
      */
-    public function getArrayCopy()
+    public function getArrayCopy(): array
     {
         return [
             'countryCode' => $this->countryCode,
@@ -228,14 +174,7 @@ class VisitLocation extends AbstractEntity implements ArraySerializableInterface
         ];
     }
 
-    /**
-     * Specify data which should be serialized to JSON
-     * @link http://php.net/manual/en/jsonserializable.jsonserialize.php
-     * @return mixed data which can be serialized by <b>json_encode</b>,
-     * which is a value of any type other than a resource.
-     * @since 5.4.0
-     */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return $this->getArrayCopy();
     }
diff --git a/module/Core/test/Service/UrlShortenerTest.php b/module/Core/test/Service/UrlShortenerTest.php
index 234240b7..44a759b7 100644
--- a/module/Core/test/Service/UrlShortenerTest.php
+++ b/module/Core/test/Service/UrlShortenerTest.php
@@ -54,7 +54,7 @@ class UrlShortenerTest extends TestCase
         $this->em->persist(Argument::any())->will(function ($arguments) {
             /** @var ShortUrl $shortUrl */
             $shortUrl = $arguments[0];
-            $shortUrl->setId(10);
+            $shortUrl->setId('10');
         });
         $repo = $this->prophesize(ObjectRepository::class);
         $repo->findOneBy(Argument::any())->willReturn(null);
diff --git a/module/Rest/src/Entity/ApiKey.php b/module/Rest/src/Entity/ApiKey.php
index fe475c3a..2246f909 100644
--- a/module/Rest/src/Entity/ApiKey.php
+++ b/module/Rest/src/Entity/ApiKey.php
@@ -23,17 +23,17 @@ class ApiKey extends AbstractEntity
      * @var string
      * @ORM\Column(name="`key`", nullable=false, unique=true)
      */
-    protected $key;
+    private $key;
     /**
      * @var \DateTime|null
      * @ORM\Column(name="expiration_date", nullable=true, type="datetime")
      */
-    protected $expirationDate;
+    private $expirationDate;
     /**
      * @var bool
      * @ORM\Column(type="boolean")
      */
-    protected $enabled;
+    private $enabled;
 
     public function __construct()
     {
@@ -41,45 +41,28 @@ class ApiKey extends AbstractEntity
         $this->key = $this->generateV4Uuid();
     }
 
-    /**
-     * @return string
-     */
     public function getKey(): string
     {
         return $this->key;
     }
 
-    /**
-     * @param string $key
-     * @return $this
-     */
-    public function setKey($key): self
+    public function setKey(string $key): self
     {
         $this->key = $key;
         return $this;
     }
 
-    /**
-     * @return \DateTime|null
-     */
     public function getExpirationDate(): ?\DateTime
     {
         return $this->expirationDate;
     }
 
-    /**
-     * @param \DateTime $expirationDate
-     * @return $this
-     */
-    public function setExpirationDate($expirationDate): self
+    public function setExpirationDate(\DateTime $expirationDate): self
     {
         $this->expirationDate = $expirationDate;
         return $this;
     }
 
-    /**
-     * @return bool
-     */
     public function isExpired(): bool
     {
         if ($this->expirationDate === null) {
@@ -89,29 +72,17 @@ class ApiKey extends AbstractEntity
         return $this->expirationDate < new \DateTime();
     }
 
-    /**
-     * @return boolean
-     */
     public function isEnabled(): bool
     {
         return $this->enabled;
     }
 
-    /**
-     * @param boolean $enabled
-     * @return $this
-     */
-    public function setEnabled($enabled): self
+    public function setEnabled(bool $enabled): self
     {
         $this->enabled = $enabled;
         return $this;
     }
 
-    /**
-     * Disables this API key
-     *
-     * @return $this
-     */
     public function disable(): self
     {
         return $this->setEnabled(false);
@@ -119,19 +90,12 @@ class ApiKey extends AbstractEntity
 
     /**
      * Tells if this api key is enabled and not expired
-     *
-     * @return bool
      */
     public function isValid(): bool
     {
         return $this->isEnabled() && ! $this->isExpired();
     }
 
-    /**
-     * The string representation of an API key is the key itself
-     *
-     * @return string
-     */
     public function __toString(): string
     {
         return $this->getKey();
diff --git a/module/Rest/test/Action/AuthenticateActionTest.php b/module/Rest/test/Action/AuthenticateActionTest.php
index 4825a100..131efb6d 100644
--- a/module/Rest/test/Action/AuthenticateActionTest.php
+++ b/module/Rest/test/Action/AuthenticateActionTest.php
@@ -55,7 +55,7 @@ class AuthenticateActionTest extends TestCase
      */
     public function properApiKeyReturnsTokenInResponse()
     {
-        $this->apiKeyService->getByKey('foo')->willReturn((new ApiKey())->setId(5))
+        $this->apiKeyService->getByKey('foo')->willReturn((new ApiKey())->setId('5'))
                                              ->shouldBeCalledTimes(1);
 
         $request = ServerRequestFactory::fromGlobals()->withParsedBody([
diff --git a/module/Rest/test/Authentication/JWTServiceTest.php b/module/Rest/test/Authentication/JWTServiceTest.php
index a80680f8..e592e865 100644
--- a/module/Rest/test/Authentication/JWTServiceTest.php
+++ b/module/Rest/test/Authentication/JWTServiceTest.php
@@ -30,7 +30,7 @@ class JWTServiceTest extends TestCase
      */
     public function tokenIsProperlyCreated()
     {
-        $id = 34;
+        $id = '34';
         $token = $this->service->create((new ApiKey())->setId($id));
         $payload = (array) JWT::decode($token, 'foo', [JWTService::DEFAULT_ENCRYPTION_ALG]);
         $this->assertGreaterThanOrEqual($payload['iat'], time());