diff --git a/CHANGELOG.md b/CHANGELOG.md
index ec4bedb3a..28dba190e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Fixed
 - Basic authorization is now case insensitive (PR #1403)
 
+### Changed
+- Request parameters are now parsed into strings to use internally in the library (PR #1402)
+
 ## [9.0.0-RC1] - released 2024-03-27
 ### Added
 - Device Authorization Grant added (PR #1074)
diff --git a/examples/src/Entities/UserEntity.php b/examples/src/Entities/UserEntity.php
index 00d038462..b5648688e 100644
--- a/examples/src/Entities/UserEntity.php
+++ b/examples/src/Entities/UserEntity.php
@@ -19,8 +19,8 @@ class UserEntity implements UserEntityInterface
     /**
      * Return the user's identifier.
      */
-    public function getIdentifier(): mixed
+    public function getIdentifier(): string
     {
-        return 1;
+        return '1';
     }
 }
diff --git a/phpcs.xml.dist b/phpcs.xml.dist
index 01f95f02b..6e471456d 100644
--- a/phpcs.xml.dist
+++ b/phpcs.xml.dist
@@ -14,6 +14,8 @@
     <file>tests</file>
     <file>examples</file>
 
+    <exclude-pattern>examples/vendor/*</exclude-pattern>
+
     <rule ref="PSR12">
         <exclude name="Generic.Files.LineLength.TooLong" />
     </rule>
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index 7bef99433..7949ac1d8 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -2,8 +2,4 @@ parameters:
     level: 8
     paths:
         - src
-        - tests
-    ignoreErrors:
-        -
-          message: '#Call to an undefined method League\\OAuth2\\Server\\ResponseTypes\\ResponseTypeInterface::getAccessToken\(\)\.#'
-          path: tests/Grant/ClientCredentialsGrantTest.php 
+        - tests
\ No newline at end of file
diff --git a/src/CryptKey.php b/src/CryptKey.php
index 944c56a05..3e1285f91 100644
--- a/src/CryptKey.php
+++ b/src/CryptKey.php
@@ -43,13 +43,13 @@ class CryptKey implements CryptKeyInterface
 
     public function __construct(string $keyPath, protected ?string $passPhrase = null, bool $keyPermissionsCheck = true)
     {
-        if (strpos($keyPath, self::FILE_PREFIX) !== 0 && $this->isValidKey($keyPath, $this->passPhrase ?? '')) {
+        if (str_starts_with($keyPath, self::FILE_PREFIX) === false && $this->isValidKey($keyPath, $this->passPhrase ?? '')) {
             $this->keyContents = $keyPath;
             $this->keyPath = '';
             // There's no file, so no need for permission check.
             $keyPermissionsCheck = false;
         } elseif (is_file($keyPath)) {
-            if (strpos($keyPath, self::FILE_PREFIX) !== 0) {
+            if (str_starts_with($keyPath, self::FILE_PREFIX) === false) {
                 $keyPath = self::FILE_PREFIX . $keyPath;
             }
 
diff --git a/src/Entities/RefreshTokenEntityInterface.php b/src/Entities/RefreshTokenEntityInterface.php
index f377cd9e1..79a99f24c 100644
--- a/src/Entities/RefreshTokenEntityInterface.php
+++ b/src/Entities/RefreshTokenEntityInterface.php
@@ -18,13 +18,17 @@ interface RefreshTokenEntityInterface
 {
     /**
      * Get the token's identifier.
+     *
+     * @return non-empty-string
      */
     public function getIdentifier(): string;
 
     /**
      * Set the token's identifier.
+     *
+     * @param non-empty-string $identifier
      */
-    public function setIdentifier(mixed $identifier): void;
+    public function setIdentifier(string $identifier): void;
 
     /**
      * Get the token's expiry date time.
diff --git a/src/Entities/ScopeEntityInterface.php b/src/Entities/ScopeEntityInterface.php
index 37c40bf55..6546d61ac 100644
--- a/src/Entities/ScopeEntityInterface.php
+++ b/src/Entities/ScopeEntityInterface.php
@@ -18,6 +18,8 @@ interface ScopeEntityInterface extends JsonSerializable
 {
     /**
      * Get the scope's identifier.
+     *
+     * @return non-empty-string
      */
     public function getIdentifier(): string;
 }
diff --git a/src/Entities/TokenInterface.php b/src/Entities/TokenInterface.php
index 96d2bd418..b9d5c270f 100644
--- a/src/Entities/TokenInterface.php
+++ b/src/Entities/TokenInterface.php
@@ -18,13 +18,17 @@ interface TokenInterface
 {
     /**
      * Get the token's identifier.
+     *
+     * @return non-empty-string
      */
     public function getIdentifier(): string;
 
     /**
      * Set the token's identifier.
+     *
+     * @param non-empty-string $identifier
      */
-    public function setIdentifier(mixed $identifier): void;
+    public function setIdentifier(string $identifier): void;
 
     /**
      * Get the token's expiry date time.
@@ -45,8 +49,10 @@ public function setUserIdentifier(string $identifier): void;
 
     /**
      * Get the token user's identifier.
+     *
+     * @return non-empty-string|null
      */
-    public function getUserIdentifier(): string|int|null;
+    public function getUserIdentifier(): string|null;
 
     /**
      * Get the client that the token was issued to.
diff --git a/src/Entities/Traits/DeviceCodeTrait.php b/src/Entities/Traits/DeviceCodeTrait.php
index ea8b2acc2..125e7ef54 100644
--- a/src/Entities/Traits/DeviceCodeTrait.php
+++ b/src/Entities/Traits/DeviceCodeTrait.php
@@ -59,6 +59,9 @@ abstract public function getExpiryDateTime(): DateTimeImmutable;
      */
     abstract public function getScopes(): array;
 
+    /**
+     * @return non-empty-string
+     */
     abstract public function getIdentifier(): string;
 
     public function getLastPolledAt(): ?DateTimeImmutable
diff --git a/src/Entities/Traits/EntityTrait.php b/src/Entities/Traits/EntityTrait.php
index 87a8f6cf6..3634b5350 100644
--- a/src/Entities/Traits/EntityTrait.php
+++ b/src/Entities/Traits/EntityTrait.php
@@ -27,7 +27,10 @@ public function getIdentifier(): string
         return $this->identifier;
     }
 
-    public function setIdentifier(mixed $identifier): void
+    /**
+     * @param non-empty-string $identifier
+     */
+    public function setIdentifier(string $identifier): void
     {
         $this->identifier = $identifier;
     }
diff --git a/src/Entities/UserEntityInterface.php b/src/Entities/UserEntityInterface.php
index ef88a84d2..0a678e735 100644
--- a/src/Entities/UserEntityInterface.php
+++ b/src/Entities/UserEntityInterface.php
@@ -16,6 +16,8 @@ interface UserEntityInterface
 {
     /**
      * Return the user's identifier.
+     *
+     * @return non-empty-string
      */
-    public function getIdentifier(): mixed;
+    public function getIdentifier(): string;
 }
diff --git a/src/Exception/OAuthServerException.php b/src/Exception/OAuthServerException.php
index 03950a381..f313967ad 100644
--- a/src/Exception/OAuthServerException.php
+++ b/src/Exception/OAuthServerException.php
@@ -269,9 +269,9 @@ public function generateHttpResponse(ResponseInterface $response, bool $useFragm
 
         if ($this->redirectUri !== null) {
             if ($useFragment === true) {
-                $this->redirectUri .= (strstr($this->redirectUri, '#') === false) ? '#' : '&';
+                $this->redirectUri .= (str_contains($this->redirectUri, '#') === false) ? '#' : '&';
             } else {
-                $this->redirectUri .= (strstr($this->redirectUri, '?') === false) ? '?' : '&';
+                $this->redirectUri .= (str_contains($this->redirectUri, '?') === false) ? '?' : '&';
             }
 
             return $response->withStatus(302)->withHeader('Location', $this->redirectUri . http_build_query($payload));
@@ -310,7 +310,7 @@ public function getHttpHeaders(): array
         // include the "WWW-Authenticate" response header field
         // matching the authentication scheme used by the client.
         if ($this->errorType === 'invalid_client' && $this->requestHasAuthorizationHeader()) {
-            $authScheme = strpos($this->serverRequest->getHeader('Authorization')[0], 'Bearer') === 0 ? 'Bearer' : 'Basic';
+            $authScheme = str_starts_with($this->serverRequest->getHeader('Authorization')[0], 'Bearer') ? 'Bearer' : 'Basic';
 
             $headers['WWW-Authenticate'] = $authScheme . ' realm="OAuth"';
         }
diff --git a/src/Grant/AbstractAuthorizeGrant.php b/src/Grant/AbstractAuthorizeGrant.php
index 4fe552c17..22b58c4e2 100644
--- a/src/Grant/AbstractAuthorizeGrant.php
+++ b/src/Grant/AbstractAuthorizeGrant.php
@@ -18,16 +18,15 @@
 use League\OAuth2\Server\RequestTypes\AuthorizationRequestInterface;
 
 use function http_build_query;
-use function strstr;
 
 abstract class AbstractAuthorizeGrant extends AbstractGrant
 {
     /**
-     * @param mixed[] $params
+     * @param array<array-key,mixed> $params
      */
     public function makeRedirectUri(string $uri, array $params = [], string $queryDelimiter = '?'): string
     {
-        $uri .= (strstr($uri, $queryDelimiter) === false) ? $queryDelimiter : '&';
+        $uri .= str_contains($uri, $queryDelimiter) ? '&' : $queryDelimiter;
 
         return $uri . http_build_query($params);
     }
diff --git a/src/Grant/AbstractGrant.php b/src/Grant/AbstractGrant.php
index 96a1fb65f..31252cc1e 100644
--- a/src/Grant/AbstractGrant.php
+++ b/src/Grant/AbstractGrant.php
@@ -49,10 +49,8 @@
 use function base64_decode;
 use function bin2hex;
 use function explode;
-use function is_null;
 use function is_string;
 use function random_bytes;
-use function strpos;
 use function substr;
 use function trim;
 
@@ -129,9 +127,9 @@ public function setRefreshTokenTTL(DateInterval $refreshTokenTTL): void
     /**
      * Set the private key
      */
-    public function setPrivateKey(CryptKeyInterface $key): void
+    public function setPrivateKey(CryptKeyInterface $privateKey): void
     {
-        $this->privateKey = $key;
+        $this->privateKey = $privateKey;
     }
 
     public function setDefaultScope(string $scope): void
@@ -139,9 +137,9 @@ public function setDefaultScope(string $scope): void
         $this->defaultScope = $scope;
     }
 
-    public function revokeRefreshTokens(bool $revokeRefreshTokens): void
+    public function revokeRefreshTokens(bool $willRevoke): void
     {
-        $this->revokeRefreshTokens = $revokeRefreshTokens;
+        $this->revokeRefreshTokens = $willRevoke;
     }
 
     /**
@@ -161,13 +159,9 @@ protected function validateClient(ServerRequestInterface $request): ClientEntity
         $client = $this->getClientEntityOrFail($clientId, $request);
 
         // If a redirect URI is provided ensure it matches what is pre-registered
-        $redirectUri = $this->getRequestParameter('redirect_uri', $request, null);
+        $redirectUri = $this->getRequestParameter('redirect_uri', $request);
 
         if ($redirectUri !== null) {
-            if (!is_string($redirectUri)) {
-                throw OAuthServerException::invalidRequest('redirect_uri');
-            }
-
             $this->validateRedirectUri($redirectUri, $client, $request);
         }
 
@@ -183,6 +177,8 @@ protected function validateClient(ServerRequestInterface $request): ClientEntity
      * doesn't actually enforce non-null returns/exception-on-no-client so
      * getClientEntity might return null. By contrast, this method will
      * always either return a ClientEntityInterface or throw.
+     *
+     * @throws OAuthServerException
      */
     protected function getClientEntityOrFail(string $clientId, ServerRequestInterface $request): ClientEntityInterface
     {
@@ -200,7 +196,8 @@ protected function getClientEntityOrFail(string $clientId, ServerRequestInterfac
      * Gets the client credentials from the request from the request body or
      * the Http Basic Authorization header
      *
-     * @return string[]
+     * @return array{0:non-empty-string,1:string}
+     * @throws OAuthServerException
      */
     protected function getClientCredentials(ServerRequestInterface $request): array
     {
@@ -208,17 +205,13 @@ protected function getClientCredentials(ServerRequestInterface $request): array
 
         $clientId = $this->getRequestParameter('client_id', $request, $basicAuthUser);
 
-        if (is_null($clientId)) {
+        if ($clientId === null) {
             throw OAuthServerException::invalidRequest('client_id');
         }
 
         $clientSecret = $this->getRequestParameter('client_secret', $request, $basicAuthPassword);
 
-        if ($clientSecret !== null && !is_string($clientSecret)) {
-            throw OAuthServerException::invalidRequest('client_secret');
-        }
-
-        return [$clientId, $clientSecret];
+        return [$clientId, $clientSecret ?? ''];
     }
 
     /**
@@ -279,19 +272,47 @@ public function validateScopes(string|array|null $scopes, string $redirectUri =
      */
     private function convertScopesQueryStringToArray(string $scopes): array
     {
-        return array_filter(explode(self::SCOPE_DELIMITER_STRING, trim($scopes)), function ($scope) {
-            return $scope !== '';
-        });
+        return array_filter(explode(self::SCOPE_DELIMITER_STRING, trim($scopes)), static fn($scope) => $scope !== '');
     }
 
     /**
-     * Retrieve request parameter.
+     * Parse request parameter.
+     *
+     * @param array<array-key, mixed> $request
+     *
+     * @return non-empty-string|null
+     * @throws OAuthServerException
      */
-    protected function getRequestParameter(string $parameter, ServerRequestInterface $request, mixed $default = null): mixed
+    private static function parseParam(string $parameter, array $request, ?string $default = null): ?string
     {
-        $requestParameters = (array) $request->getParsedBody();
+        $value = $request[$parameter] ?? '';
+
+        if (is_scalar($value)) {
+            $value = trim((string)$value);
+        } else {
+            throw OAuthServerException::invalidRequest($parameter);
+        }
+
+        if ($value === '') {
+            $value = $default === null ? null : trim($default);
+
+            if ($value === '') {
+                $value = null;
+            }
+        }
 
-        return $requestParameters[$parameter] ?? $default;
+        return $value;
+    }
+
+    /**
+     * Retrieve request parameter.
+     *
+     * @return non-empty-string|null
+     * @throws OAuthServerException
+     */
+    protected function getRequestParameter(string $parameter, ServerRequestInterface $request, ?string $default = null): ?string
+    {
+        return self::parseParam($parameter, (array) $request->getParsedBody(), $default);
     }
 
     /**
@@ -301,7 +322,7 @@ protected function getRequestParameter(string $parameter, ServerRequestInterface
      * not exist, or is otherwise an invalid HTTP Basic header, return
      * [null, null].
      *
-     * @return string[]|null[]
+     * @return array{0:non-empty-string,1:string}|array{0:null,1:null}
      */
     protected function getBasicAuthCredentials(ServerRequestInterface $request): array
     {
@@ -320,35 +341,50 @@ protected function getBasicAuthCredentials(ServerRequestInterface $request): arr
             return [null, null];
         }
 
-        if (strpos($decoded, ':') === false) {
+        if (str_contains($decoded, ':') === false) {
             return [null, null]; // HTTP Basic header without colon isn't valid
         }
 
-        return explode(':', $decoded, 2);
+        [$username, $password] = explode(':', $decoded, 2);
+
+        if ($username === '') {
+            return [null, null];
+        }
+
+        return [$username, $password];
     }
 
     /**
      * Retrieve query string parameter.
+     *
+     * @return non-empty-string|null
+     * @throws OAuthServerException
      */
-    protected function getQueryStringParameter(string $parameter, ServerRequestInterface $request, mixed $default = null): ?string
+    protected function getQueryStringParameter(string $parameter, ServerRequestInterface $request, ?string $default = null): ?string
     {
-        return isset($request->getQueryParams()[$parameter]) ? $request->getQueryParams()[$parameter] : $default;
+        return self::parseParam($parameter, $request->getQueryParams(), $default);
     }
 
     /**
      * Retrieve cookie parameter.
+     *
+     * @return non-empty-string|null
+     * @throws OAuthServerException
      */
-    protected function getCookieParameter(string $parameter, ServerRequestInterface $request, mixed $default = null): ?string
+    protected function getCookieParameter(string $parameter, ServerRequestInterface $request, ?string $default = null): ?string
     {
-        return isset($request->getCookieParams()[$parameter]) ? $request->getCookieParams()[$parameter] : $default;
+        return self::parseParam($parameter, $request->getCookieParams(), $default);
     }
 
     /**
      * Retrieve server parameter.
+     *
+     * @return non-empty-string|null
+     * @throws OAuthServerException
      */
-    protected function getServerParameter(string $parameter, ServerRequestInterface $request, mixed $default = null): ?string
+    protected function getServerParameter(string $parameter, ServerRequestInterface $request, ?string $default = null): ?string
     {
-        return isset($request->getServerParams()[$parameter]) ? $request->getServerParams()[$parameter] : $default;
+        return self::parseParam($parameter, $request->getServerParams(), $default);
     }
 
     /**
@@ -362,7 +398,7 @@ protected function getServerParameter(string $parameter, ServerRequestInterface
     protected function issueAccessToken(
         DateInterval $accessTokenTTL,
         ClientEntityInterface $client,
-        string|int|null $userIdentifier,
+        string|null $userIdentifier,
         array $scopes = []
     ): AccessTokenEntityInterface {
         $maxGenerationAttempts = self::MAX_RANDOM_TOKEN_GENERATION_ATTEMPTS;
@@ -473,6 +509,8 @@ protected function issueRefreshToken(AccessTokenEntityInterface $accessToken): ?
     /**
      * Generate a new unique identifier.
      *
+     * @return non-empty-string
+     *
      * @throws OAuthServerException
      */
     protected function generateUniqueIdentifier(int $length = 40): string
diff --git a/src/Grant/AuthCodeGrant.php b/src/Grant/AuthCodeGrant.php
index 83bddac18..8a24a8e95 100644
--- a/src/Grant/AuthCodeGrant.php
+++ b/src/Grant/AuthCodeGrant.php
@@ -41,7 +41,6 @@
 use function implode;
 use function in_array;
 use function is_array;
-use function is_string;
 use function json_decode;
 use function json_encode;
 use function preg_match;
@@ -106,9 +105,9 @@ public function respondToAccessTokenRequest(
             $this->validateClient($request);
         }
 
-        $encryptedAuthCode = $this->getRequestParameter('code', $request, null);
+        $encryptedAuthCode = $this->getRequestParameter('code', $request);
 
-        if (!is_string($encryptedAuthCode)) {
+        if ($encryptedAuthCode === null) {
             throw OAuthServerException::invalidRequest('code');
         }
 
@@ -128,7 +127,7 @@ public function respondToAccessTokenRequest(
             throw OAuthServerException::invalidRequest('code', 'Cannot decrypt the authorization code', $e);
         }
 
-        $codeVerifier = $this->getRequestParameter('code_verifier', $request, null);
+        $codeVerifier = $this->getRequestParameter('code_verifier', $request);
 
         // If a code challenge isn't present but a code verifier is, reject the request to block PKCE downgrade attack
         if (!isset($authCodePayload->code_challenge) && $codeVerifier !== null) {
@@ -219,7 +218,7 @@ private function validateAuthorizationCode(
         }
 
         // The redirect URI is required in this request
-        $redirectUri = $this->getRequestParameter('redirect_uri', $request, null);
+        $redirectUri = $this->getRequestParameter('redirect_uri', $request);
         if ($authCodePayload->redirect_uri !== '' && $redirectUri === null) {
             throw OAuthServerException::invalidRequest('redirect_uri');
         }
diff --git a/src/Grant/DeviceCodeGrant.php b/src/Grant/DeviceCodeGrant.php
index 89568a19b..3804e2027 100644
--- a/src/Grant/DeviceCodeGrant.php
+++ b/src/Grant/DeviceCodeGrant.php
@@ -153,10 +153,10 @@ public function respondToAccessTokenRequest(
         }
 
         // Finalize the requested scopes
-        $finalizedScopes = $this->scopeRepository->finalizeScopes($scopes, $this->getIdentifier(), $client, (string) $deviceCodeEntity->getUserIdentifier());
+        $finalizedScopes = $this->scopeRepository->finalizeScopes($scopes, $this->getIdentifier(), $client, $deviceCodeEntity->getUserIdentifier());
 
         // Issue and persist new access token
-        $accessToken = $this->issueAccessToken($accessTokenTTL, $client, (string) $deviceCodeEntity->getUserIdentifier(), $finalizedScopes);
+        $accessToken = $this->issueAccessToken($accessTokenTTL, $client, $deviceCodeEntity->getUserIdentifier(), $finalizedScopes);
         $this->getEmitter()->emit(new RequestEvent(RequestEvent::ACCESS_TOKEN_ISSUED, $request));
         $responseType->setAccessToken($accessToken);
 
@@ -304,9 +304,7 @@ protected function generateUserCode(int $length = 8): string
 
             return $userCode;
             // @codeCoverageIgnoreStart
-        } catch (TypeError $e) {
-            throw OAuthServerException::serverError('An unexpected error has occurred', $e);
-        } catch (Error $e) {
+        } catch (TypeError | Error $e) {
             throw OAuthServerException::serverError('An unexpected error has occurred', $e);
         } catch (Exception $e) {
             // If you get this message, the CSPRNG failed hard.
diff --git a/src/Grant/PasswordGrant.php b/src/Grant/PasswordGrant.php
index 1b92dc1da..f5d8da322 100644
--- a/src/Grant/PasswordGrant.php
+++ b/src/Grant/PasswordGrant.php
@@ -26,8 +26,6 @@
 use League\OAuth2\Server\ResponseTypes\ResponseTypeInterface;
 use Psr\Http\Message\ServerRequestInterface;
 
-use function is_string;
-
 /**
  * Password grant class.
  */
@@ -84,17 +82,11 @@ public function respondToAccessTokenRequest(
      */
     protected function validateUser(ServerRequestInterface $request, ClientEntityInterface $client): UserEntityInterface
     {
-        $username = $this->getRequestParameter('username', $request);
-
-        if (!is_string($username)) {
-            throw OAuthServerException::invalidRequest('username');
-        }
+        $username = $this->getRequestParameter('username', $request)
+            ?? throw OAuthServerException::invalidRequest('username');
 
-        $password = $this->getRequestParameter('password', $request);
-
-        if (!is_string($password)) {
-            throw OAuthServerException::invalidRequest('password');
-        }
+        $password = $this->getRequestParameter('password', $request)
+            ?? throw OAuthServerException::invalidRequest('password');
 
         $user = $this->userRepository->getUserEntityByUserCredentials(
             $username,
diff --git a/src/Grant/RefreshTokenGrant.php b/src/Grant/RefreshTokenGrant.php
index 5a5b55e30..a632990c7 100644
--- a/src/Grant/RefreshTokenGrant.php
+++ b/src/Grant/RefreshTokenGrant.php
@@ -26,7 +26,6 @@
 
 use function implode;
 use function in_array;
-use function is_string;
 use function json_decode;
 use function time;
 
@@ -53,6 +52,7 @@ public function respondToAccessTokenRequest(
         // Validate request
         $client = $this->validateClient($request);
         $oldRefreshToken = $this->validateOldRefreshToken($request, $client->getIdentifier());
+
         $scopes = $this->validateScopes(
             $this->getRequestParameter(
                 'scope',
@@ -102,10 +102,8 @@ public function respondToAccessTokenRequest(
      */
     protected function validateOldRefreshToken(ServerRequestInterface $request, string $clientId): array
     {
-        $encryptedRefreshToken = $this->getRequestParameter('refresh_token', $request);
-        if (!is_string($encryptedRefreshToken)) {
-            throw OAuthServerException::invalidRequest('refresh_token');
-        }
+        $encryptedRefreshToken = $this->getRequestParameter('refresh_token', $request)
+            ?? throw OAuthServerException::invalidRequest('refresh_token');
 
         // Validate refresh token
         try {
diff --git a/src/Repositories/AccessTokenRepositoryInterface.php b/src/Repositories/AccessTokenRepositoryInterface.php
index d392716ed..8bac8be64 100644
--- a/src/Repositories/AccessTokenRepositoryInterface.php
+++ b/src/Repositories/AccessTokenRepositoryInterface.php
@@ -30,7 +30,7 @@ interface AccessTokenRepositoryInterface extends RepositoryInterface
     public function getNewToken(
         ClientEntityInterface $clientEntity,
         array $scopes,
-        mixed $userIdentifier = null
+        string|null $userIdentifier = null
     ): AccessTokenEntityInterface;
 
     /**
diff --git a/src/Repositories/ScopeRepositoryInterface.php b/src/Repositories/ScopeRepositoryInterface.php
index 95bfdbb9a..e5ae7c716 100644
--- a/src/Repositories/ScopeRepositoryInterface.php
+++ b/src/Repositories/ScopeRepositoryInterface.php
@@ -39,7 +39,7 @@ public function finalizeScopes(
         array $scopes,
         string $grantType,
         ClientEntityInterface $clientEntity,
-        string|int|null $userIdentifier = null,
+        string|null $userIdentifier = null,
         ?string $authCodeId = null
     ): array;
 }
diff --git a/src/ResponseTypes/BearerTokenResponse.php b/src/ResponseTypes/BearerTokenResponse.php
index c33bdc712..dd49b99ba 100644
--- a/src/ResponseTypes/BearerTokenResponse.php
+++ b/src/ResponseTypes/BearerTokenResponse.php
@@ -73,7 +73,7 @@ public function generateHttpResponse(ResponseInterface $response): ResponseInter
      * AuthorizationServer::getResponseType() to pull in your version of
      * this class rather than the default.
      *
-     * @return mixed[]
+     * @return array<array-key,mixed>
      */
     protected function getExtraParams(AccessTokenEntityInterface $accessToken): array
     {
diff --git a/src/ResponseTypes/DeviceCodeResponse.php b/src/ResponseTypes/DeviceCodeResponse.php
index 339c75bd3..91a8df69a 100644
--- a/src/ResponseTypes/DeviceCodeResponse.php
+++ b/src/ResponseTypes/DeviceCodeResponse.php
@@ -84,7 +84,7 @@ public function includeVerificationUriComplete(): void
      * AuthorizationServer::getResponseType() to pull in your version of
      * this class rather than the default.
      *
-     * @return mixed[]
+     * @return array<array-key,mixed>
      */
     protected function getExtraParams(DeviceCodeEntityInterface $deviceCode): array
     {
diff --git a/tests/Exception/OAuthServerExceptionTest.php b/tests/Exception/OAuthServerExceptionTest.php
index 27922b427..93db59f2a 100644
--- a/tests/Exception/OAuthServerExceptionTest.php
+++ b/tests/Exception/OAuthServerExceptionTest.php
@@ -123,7 +123,7 @@ public function testHasPrevious(): void
         $previous = new Exception('This is the previous');
         $exceptionWithPrevious = OAuthServerException::accessDenied(null, null, $previous);
 
-        $previousMessage = $exceptionWithPrevious->getPrevious() !== null ? $exceptionWithPrevious->getPrevious()->getMessage() : null;
+        $previousMessage = $exceptionWithPrevious->getPrevious()?->getMessage();
 
         self::assertSame('This is the previous', $previousMessage);
     }
diff --git a/tests/Grant/AuthCodeGrantTest.php b/tests/Grant/AuthCodeGrantTest.php
index 4a3c1e935..6a6842661 100644
--- a/tests/Grant/AuthCodeGrantTest.php
+++ b/tests/Grant/AuthCodeGrantTest.php
@@ -602,7 +602,7 @@ public function testRespondToAccessTokenRequest(): void
                         'auth_code_id' => uniqid(),
                         'expire_time'  => time() + 3600,
                         'client_id'    => 'foo',
-                        'user_id'      => 123,
+                        'user_id'      => '123',
                         'scopes'       => ['foo'],
                         'redirect_uri' => self::REDIRECT_URI,
                     ], JSON_THROW_ON_ERROR)
@@ -665,7 +665,7 @@ public function testRespondToAccessTokenRequestUsingHttpBasicAuth(): void
                         'auth_code_id' => uniqid(),
                         'client_id' => 'foo',
                         'expire_time'  => time() + 3600,
-                        'user_id'      => 123,
+                        'user_id'      => '123',
                         'scopes'       => ['foo'],
                         'redirect_uri' => self::REDIRECT_URI,
                     ], JSON_THROW_ON_ERROR)
@@ -730,7 +730,7 @@ public function testRespondToAccessTokenRequestForPublicClient(): void
                         'auth_code_id' => uniqid(),
                         'expire_time'  => time() + 3600,
                         'client_id'    => 'foo',
-                        'user_id'      => 123,
+                        'user_id'      => '123',
                         'scopes'       => ['foo'],
                         'redirect_uri' => self::REDIRECT_URI,
                     ], JSON_THROW_ON_ERROR)
@@ -795,7 +795,7 @@ public function testRespondToAccessTokenRequestNullRefreshToken(): void
                         'auth_code_id' => uniqid(),
                         'expire_time'  => time() + 3600,
                         'client_id'    => 'foo',
-                        'user_id'      => 123,
+                        'user_id'      => '123',
                         'scopes'       => ['foo'],
                         'redirect_uri' => self::REDIRECT_URI,
                     ], JSON_THROW_ON_ERROR)
@@ -867,7 +867,7 @@ public function testRespondToAccessTokenRequestCodeChallengePlain(): void
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => self::CODE_VERIFIER,
@@ -941,7 +941,7 @@ public function testRespondToAccessTokenRequestCodeChallengeS256(): void
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => self::CODE_CHALLENGE,
@@ -1013,7 +1013,7 @@ public function testPKCEDowngradeBlocked(): void
                             'auth_code_id'          => uniqid(),
                             'expire_time'           => time() + 3600,
                             'client_id'             => 'foo',
-                            'user_id'               => 123,
+                            'user_id'               => '123',
                             'scopes'                => ['foo'],
                             'redirect_uri'          => self::REDIRECT_URI,
                         ],
@@ -1561,7 +1561,7 @@ public function testRespondToAccessTokenRequestBadCodeVerifierPlain(): void
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => 'foobar',
@@ -1636,7 +1636,7 @@ public function testRespondToAccessTokenRequestBadCodeVerifierS256(): void
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => 'foobar',
@@ -1711,7 +1711,7 @@ public function testRespondToAccessTokenRequestMalformedCodeVerifierS256WithInva
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => self::CODE_CHALLENGE,
@@ -1786,7 +1786,7 @@ public function testRespondToAccessTokenRequestMalformedCodeVerifierS256WithInva
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => 'R7T1y1HPNFvs1WDCrx4lfoBS6KD2c71pr8OHvULjvv8',
@@ -1860,7 +1860,7 @@ public function testRespondToAccessTokenRequestMissingCodeVerifier(): void
                         'auth_code_id'          => uniqid(),
                         'expire_time'           => time() + 3600,
                         'client_id'             => 'foo',
-                        'user_id'               => 123,
+                        'user_id'               => '123',
                         'scopes'                => ['foo'],
                         'redirect_uri'          => self::REDIRECT_URI,
                         'code_challenge'        => 'foobar',
@@ -2034,7 +2034,7 @@ public function testRefreshTokenRepositoryUniqueConstraintCheck(): void
                         'auth_code_id' => uniqid(),
                         'expire_time'  => time() + 3600,
                         'client_id'    => 'foo',
-                        'user_id'      => 123,
+                        'user_id'      => '123',
                         'scopes'       => ['foo'],
                         'redirect_uri' => self::REDIRECT_URI,
                     ], JSON_THROW_ON_ERROR)
@@ -2099,7 +2099,7 @@ public function testRefreshTokenRepositoryFailToPersist(): void
                         'auth_code_id' => uniqid(),
                         'expire_time'  => time() + 3600,
                         'client_id'    => 'foo',
-                        'user_id'      => 123,
+                        'user_id'      => '123',
                         'scopes'       => ['foo'],
                         'redirect_uri' => self::REDIRECT_URI,
                     ], JSON_THROW_ON_ERROR)
@@ -2167,7 +2167,7 @@ public function testRefreshTokenRepositoryFailToPersistUniqueNoInfiniteLoop(): v
                         'auth_code_id' => uniqid(),
                         'expire_time'  => time() + 3600,
                         'client_id'    => 'foo',
-                        'user_id'      => 123,
+                        'user_id'      => '123',
                         'scopes'       => ['foo'],
                         'redirect_uri' => self::REDIRECT_URI,
                     ], JSON_THROW_ON_ERROR)
diff --git a/tests/Grant/ClientCredentialsGrantTest.php b/tests/Grant/ClientCredentialsGrantTest.php
index 264e026e2..69f756c37 100644
--- a/tests/Grant/ClientCredentialsGrantTest.php
+++ b/tests/Grant/ClientCredentialsGrantTest.php
@@ -59,6 +59,8 @@ public function testRespondToRequest(): void
         ]);
 
         $responseType = new StubResponseType();
+
+        /** @var StubResponseType $response */
         $response = $grant->respondToAccessTokenRequest($serverRequest, $responseType, new DateInterval('PT5M'));
 
         self::assertNotEmpty($response->getAccessToken()->getIdentifier());
diff --git a/tests/Grant/ImplicitGrantTest.php b/tests/Grant/ImplicitGrantTest.php
index 515629247..c2b943197 100644
--- a/tests/Grant/ImplicitGrantTest.php
+++ b/tests/Grant/ImplicitGrantTest.php
@@ -23,6 +23,7 @@
 use LeagueTests\Stubs\StubResponseType;
 use LeagueTests\Stubs\UserEntity;
 use LogicException;
+use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
 
 class ImplicitGrantTest extends TestCase
@@ -282,7 +283,7 @@ public function testAccessTokenRepositoryUniqueConstraintCheck(): void
         $accessToken->setClient($client);
         $accessToken->setUserIdentifier('userId');
 
-        /** @var AccessTokenRepositoryInterface|\PHPUnit\Framework\MockObject\MockObject $accessTokenRepositoryMock */
+        /** @var AccessTokenRepositoryInterface|MockObject $accessTokenRepositoryMock */
         $accessTokenRepositoryMock = $this->getMockBuilder(AccessTokenRepositoryInterface::class)->getMock();
         $accessTokenRepositoryMock->method('getNewToken')->willReturn($accessToken);
 
@@ -321,7 +322,7 @@ public function testAccessTokenRepositoryFailToPersist(): void
         $authRequest->setGrantTypeId('authorization_code');
         $authRequest->setUser(new UserEntity());
 
-        /** @var AccessTokenRepositoryInterface|\PHPUnit\Framework\MockObject\MockObject $accessTokenRepositoryMock */
+        /** @var AccessTokenRepositoryInterface|MockObject $accessTokenRepositoryMock */
         $accessTokenRepositoryMock = $this->getMockBuilder(AccessTokenRepositoryInterface::class)->getMock();
         $accessTokenRepositoryMock->method('getNewToken')->willReturn(new AccessTokenEntity());
         $accessTokenRepositoryMock->method('persistNewAccessToken')->willThrowException(OAuthServerException::serverError('something bad happened'));
@@ -353,7 +354,7 @@ public function testAccessTokenRepositoryFailToPersistUniqueNoInfiniteLoop(): vo
         $authRequest->setGrantTypeId('authorization_code');
         $authRequest->setUser(new UserEntity());
 
-        /** @var AccessTokenRepositoryInterface|\PHPUnit\Framework\MockObject\MockObject $accessTokenRepositoryMock */
+        /** @var AccessTokenRepositoryInterface|MockObject $accessTokenRepositoryMock */
         $accessTokenRepositoryMock = $this->getMockBuilder(AccessTokenRepositoryInterface::class)->getMock();
         $accessTokenRepositoryMock->method('getNewToken')->willReturn(new AccessTokenEntity());
         $accessTokenRepositoryMock->method('persistNewAccessToken')->willThrowException(UniqueTokenIdentifierConstraintViolationException::create());
diff --git a/tests/Grant/RefreshTokenGrantTest.php b/tests/Grant/RefreshTokenGrantTest.php
index 747ba5ead..b37001a80 100644
--- a/tests/Grant/RefreshTokenGrantTest.php
+++ b/tests/Grant/RefreshTokenGrantTest.php
@@ -80,7 +80,7 @@ public function testRespondToRequest(): void
                 'refresh_token_id' => 'zyxwvu',
                 'access_token_id'  => 'abcdef',
                 'scopes'           => ['foo'],
-                'user_id'          => 123,
+                'user_id'          => '123',
                 'expire_time'      => time() + 3600,
             ]
         );
@@ -144,7 +144,7 @@ public function testRespondToRequestNullRefreshToken(): void
                 'refresh_token_id' => 'zyxwvu',
                 'access_token_id'  => 'abcdef',
                 'scopes'           => ['foo'],
-                'user_id'          => 123,
+                'user_id'          => '123',
                 'expire_time'      => time() + 3600,
             ]
         );
@@ -208,7 +208,7 @@ public function testRespondToReducedScopes(): void
                 'refresh_token_id' => 'zyxwvu',
                 'access_token_id'  => 'abcdef',
                 'scopes'           => ['foo', 'bar'],
-                'user_id'          => 123,
+                'user_id'          => '123',
                 'expire_time'      => time() + 3600,
             ]
         );
@@ -578,7 +578,7 @@ public function testRespondToRequestFinalizeScopes(): void
                 'refresh_token_id' => 'zyxwvu',
                 'access_token_id'  => 'abcdef',
                 'scopes'           => ['foo', 'bar'],
-                'user_id'          => 123,
+                'user_id'          => '123',
                 'expire_time'      => time() + 3600,
             ]
         );
@@ -595,7 +595,7 @@ public function testRespondToRequestFinalizeScopes(): void
            'client_id'     => 'foo',
            'client_secret' => 'bar',
            'refresh_token' => $encryptedOldRefreshToken,
-           'scope'         =>  ['foo', 'bar'],
+           'scope'         =>  'foo bar',
         ]);
 
         $responseType = new StubResponseType();
@@ -637,7 +637,7 @@ public function testRevokedRefreshToken(): void
                 'refresh_token_id' => $refreshTokenId,
                 'access_token_id'  => 'abcdef',
                 'scopes'           => ['foo'],
-                'user_id'          => 123,
+                'user_id'          => '123',
                 'expire_time'      => time() + 3600,
             ]
         );
@@ -654,7 +654,7 @@ public function testRevokedRefreshToken(): void
             'client_id'     => 'foo',
             'client_secret' => 'bar',
             'refresh_token' => $encryptedOldRefreshToken,
-            'scope'         => ['foo'],
+            'scope'         => 'foo',
         ]);
 
         $grant = new RefreshTokenGrant($refreshTokenRepositoryMock);
@@ -702,7 +702,7 @@ public function testUnrevokedRefreshToken(): void
                 'refresh_token_id' => $refreshTokenId,
                 'access_token_id'  => 'abcdef',
                 'scopes'           => ['foo'],
-                'user_id'          => 123,
+                'user_id'          => '123',
                 'expire_time'      => time() + 3600,
             ]
         );
@@ -719,7 +719,7 @@ public function testUnrevokedRefreshToken(): void
             'client_id'     => 'foo',
             'client_secret' => 'bar',
             'refresh_token' => $encryptedOldRefreshToken,
-            'scope'         => ['foo'],
+            'scope'         => 'foo',
         ]);
 
         $grant = new RefreshTokenGrant($refreshTokenRepositoryMock);
diff --git a/tests/Stubs/StubResponseType.php b/tests/Stubs/StubResponseType.php
index 0f8f371a1..02f6f14e8 100644
--- a/tests/Stubs/StubResponseType.php
+++ b/tests/Stubs/StubResponseType.php
@@ -35,7 +35,7 @@ public function setRefreshToken(RefreshTokenEntityInterface $refreshToken): void
     }
 
     /**
-     * @throws \League\OAuth2\Server\Exception\OAuthServerException
+     * @throws OAuthServerException
      */
     public function validateAccessToken(ServerRequestInterface $request): ServerRequestInterface
     {