diff --git a/src/Executor/ExecutionResult.php b/src/Executor/ExecutionResult.php
index 043ddc8..dbc3591 100644
--- a/src/Executor/ExecutionResult.php
+++ b/src/Executor/ExecutionResult.php
@@ -78,7 +78,7 @@ class ExecutionResult implements \JsonSerializable
      * );
      *
      * @api
-     * @return $this
+     * @return self
      */
     public function setErrorFormatter(callable $errorFormatter)
     {
@@ -98,7 +98,7 @@ class ExecutionResult implements \JsonSerializable
      * }
      *
      * @api
-     * @return $this
+     * @return self
      */
     public function setErrorsHandler(callable $handler)
     {
diff --git a/src/Language/AST/Node.php b/src/Language/AST/Node.php
index e12deec..4645c4f 100644
--- a/src/Language/AST/Node.php
+++ b/src/Language/AST/Node.php
@@ -48,7 +48,7 @@ abstract class Node
     }
 
     /**
-     * @return $this
+     * @return self
      */
     public function cloneDeep()
     {
diff --git a/src/Language/Lexer.php b/src/Language/Lexer.php
index 924f8e0..e39d933 100644
--- a/src/Language/Lexer.php
+++ b/src/Language/Lexer.php
@@ -690,7 +690,7 @@ class Lexer
      *
      * @param $positionOffset
      * @param $byteStreamOffset
-     * @return $this
+     * @return self
      */
     private function moveStringCursor($positionOffset, $byteStreamOffset)
     {
diff --git a/src/Server/Helper.php b/src/Server/Helper.php
index d39de0f..552f449 100644
--- a/src/Server/Helper.php
+++ b/src/Server/Helper.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 namespace GraphQL\Server;
 
 use GraphQL\Error\Error;
@@ -17,13 +20,23 @@ use GraphQL\Utils\Utils;
 use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\ServerRequestInterface;
 use Psr\Http\Message\StreamInterface;
+use function file_get_contents;
+use function header;
+use function is_array;
+use function is_callable;
+use function is_string;
+use function json_decode;
+use function json_encode;
+use function json_last_error;
+use function json_last_error_msg;
+use function sprintf;
+use function stripos;
 
 /**
  * Contains functionality that could be re-used by various server implementations
  */
 class Helper
 {
-
     /**
      * Parses HTTP request using PHP globals and returns GraphQL OperationParams
      * contained in this request. For batched requests it returns an array of OperationParams.
@@ -40,43 +53,45 @@ class Helper
      * For PSR-7 request parsing use `parsePsrRequest()` instead.
      *
      * @api
-     * @param callable|null $readRawBodyFn
      * @return OperationParams|OperationParams[]
      * @throws RequestError
      */
-    public function parseHttpRequest(callable $readRawBodyFn = null)
+    public function parseHttpRequest(?callable $readRawBodyFn = null)
     {
-        $method = isset($_SERVER['REQUEST_METHOD']) ? $_SERVER['REQUEST_METHOD'] : null;
+        $method     = $_SERVER['REQUEST_METHOD'] ?? null;
         $bodyParams = [];
-        $urlParams = $_GET;
+        $urlParams  = $_GET;
 
         if ($method === 'POST') {
-            $contentType = isset($_SERVER['CONTENT_TYPE']) ? $_SERVER['CONTENT_TYPE'] : null;
+            $contentType = $_SERVER['CONTENT_TYPE'] ?? null;
+
+            if ($contentType === null) {
+                throw new RequestError('Missing "Content-Type" header');
+            }
 
             if (stripos($contentType, 'application/graphql') !== false) {
-                $rawBody =  $readRawBodyFn ? $readRawBodyFn() : $this->readRawBody();
+                $rawBody    = $readRawBodyFn ? $readRawBodyFn() : $this->readRawBody();
                 $bodyParams = ['query' => $rawBody ?: ''];
-            } else if (stripos($contentType, 'application/json') !== false) {
-                $rawBody = $readRawBodyFn ? $readRawBodyFn() : $this->readRawBody();
+            } elseif (stripos($contentType, 'application/json') !== false) {
+                $rawBody    = $readRawBodyFn ? $readRawBodyFn() : $this->readRawBody();
                 $bodyParams = json_decode($rawBody ?: '', true);
 
                 if (json_last_error()) {
-                    throw new RequestError("Could not parse JSON: " . json_last_error_msg());
+                    throw new RequestError('Could not parse JSON: ' . json_last_error_msg());
                 }
-                if (!is_array($bodyParams)) {
+
+                if (! is_array($bodyParams)) {
                     throw new RequestError(
-                        "GraphQL Server expects JSON object or array, but got " .
+                        'GraphQL Server expects JSON object or array, but got ' .
                         Utils::printSafeJson($bodyParams)
                     );
                 }
-            } else if (stripos($contentType, 'application/x-www-form-urlencoded') !== false) {
+            } elseif (stripos($contentType, 'application/x-www-form-urlencoded') !== false) {
                 $bodyParams = $_POST;
-            } else if (stripos($contentType, 'multipart/form-data') !== false) {
+            } elseif (stripos($contentType, 'multipart/form-data') !== false) {
                 $bodyParams = $_POST;
-            } else if (null === $contentType) {
-                throw new RequestError('Missing "Content-Type" header');
             } else {
-                throw new RequestError("Unexpected content type: " . Utils::printSafeJson($contentType));
+                throw new RequestError('Unexpected content type: ' . Utils::printSafeJson($contentType));
             }
         }
 
@@ -90,9 +105,9 @@ class Helper
      * Returned value is a suitable input for `executeOperation` or `executeBatch` (if array)
      *
      * @api
-     * @param string $method
-     * @param array $bodyParams
-     * @param array $queryParams
+     * @param string  $method
+     * @param mixed[] $bodyParams
+     * @param mixed[] $queryParams
      * @return OperationParams|OperationParams[]
      * @throws RequestError
      */
@@ -100,11 +115,11 @@ class Helper
     {
         if ($method === 'GET') {
             $result = OperationParams::create($queryParams, true);
-        } else if ($method === 'POST') {
+        } elseif ($method === 'POST') {
             if (isset($bodyParams[0])) {
                 $result = [];
                 foreach ($bodyParams as $index => $entry) {
-                    $op = OperationParams::create($entry);
+                    $op       = OperationParams::create($entry);
                     $result[] = $op;
                 }
             } else {
@@ -113,6 +128,7 @@ class Helper
         } else {
             throw new RequestError('HTTP Method "' . $method . '" is not supported');
         }
+
         return $result;
     }
 
@@ -121,44 +137,47 @@ class Helper
      * if params are invalid (or empty array when params are valid)
      *
      * @api
-     * @param OperationParams $params
      * @return Error[]
      */
     public function validateOperationParams(OperationParams $params)
     {
         $errors = [];
-        if (!$params->query && !$params->queryId) {
+        if (! $params->query && ! $params->queryId) {
             $errors[] = new RequestError('GraphQL Request must include at least one of those two parameters: "query" or "queryId"');
         }
+
         if ($params->query && $params->queryId) {
             $errors[] = new RequestError('GraphQL Request parameters "query" and "queryId" are mutually exclusive');
         }
 
-        if ($params->query !== null && (!is_string($params->query) || empty($params->query))) {
+        if ($params->query !== null && (! is_string($params->query) || empty($params->query))) {
             $errors[] = new RequestError(
                 'GraphQL Request parameter "query" must be string, but got ' .
                 Utils::printSafeJson($params->query)
             );
         }
-        if ($params->queryId !== null && (!is_string($params->queryId) || empty($params->queryId))) {
+
+        if ($params->queryId !== null && (! is_string($params->queryId) || empty($params->queryId))) {
             $errors[] = new RequestError(
                 'GraphQL Request parameter "queryId" must be string, but got ' .
                 Utils::printSafeJson($params->queryId)
             );
         }
 
-        if ($params->operation !== null && (!is_string($params->operation) || empty($params->operation))) {
+        if ($params->operation !== null && (! is_string($params->operation) || empty($params->operation))) {
             $errors[] = new RequestError(
                 'GraphQL Request parameter "operation" must be string, but got ' .
                 Utils::printSafeJson($params->operation)
             );
         }
-        if ($params->variables !== null && (!is_array($params->variables) || isset($params->variables[0]))) {
+
+        if ($params->variables !== null && (! is_array($params->variables) || isset($params->variables[0]))) {
             $errors[] = new RequestError(
                 'GraphQL Request parameter "variables" must be object or JSON string parsed to object, but got ' .
                 Utils::printSafeJson($params->getOriginalInput('variables'))
             );
         }
+
         return $errors;
     }
 
@@ -167,15 +186,13 @@ class Helper
      * (or promise when promise adapter is different from SyncPromiseAdapter)
      *
      * @api
-     * @param ServerConfig $config
-     * @param OperationParams $op
      *
      * @return ExecutionResult|Promise
      */
     public function executeOperation(ServerConfig $config, OperationParams $op)
     {
         $promiseAdapter = $config->getPromiseAdapter() ?: Executor::getPromiseAdapter();
-        $result = $this->promiseToExecuteOperation($promiseAdapter, $config, $op);
+        $result         = $this->promiseToExecuteOperation($promiseAdapter, $config, $op);
 
         if ($promiseAdapter instanceof SyncPromiseAdapter) {
             $result = $promiseAdapter->wait($result);
@@ -189,14 +206,13 @@ class Helper
      * (thus, effectively batching deferreds|promises of all queries at once)
      *
      * @api
-     * @param ServerConfig $config
      * @param OperationParams[] $operations
      * @return ExecutionResult|ExecutionResult[]|Promise
      */
     public function executeBatch(ServerConfig $config, array $operations)
     {
         $promiseAdapter = $config->getPromiseAdapter() ?: Executor::getPromiseAdapter();
-        $result = [];
+        $result         = [];
 
         foreach ($operations as $operation) {
             $result[] = $this->promiseToExecuteOperation($promiseAdapter, $config, $operation, true);
@@ -208,32 +224,39 @@ class Helper
         if ($promiseAdapter instanceof SyncPromiseAdapter) {
             $result = $promiseAdapter->wait($result);
         }
+
         return $result;
     }
 
     /**
-     * @param PromiseAdapter $promiseAdapter
-     * @param ServerConfig $config
-     * @param OperationParams $op
      * @param bool $isBatch
      * @return Promise
      */
-    private function promiseToExecuteOperation(PromiseAdapter $promiseAdapter, ServerConfig $config, OperationParams $op, $isBatch = false)
-    {
+    private function promiseToExecuteOperation(
+        PromiseAdapter $promiseAdapter,
+        ServerConfig $config,
+        OperationParams $op,
+        $isBatch = false
+    ) {
         try {
-            if (!$config->getSchema()) {
-                throw new InvariantViolation("Schema is required for the server");
+            if (! $config->getSchema()) {
+                throw new InvariantViolation('Schema is required for the server');
             }
-            if ($isBatch && !$config->getQueryBatching()) {
-                throw new RequestError("Batched queries are not supported by this server");
+
+            if ($isBatch && ! $config->getQueryBatching()) {
+                throw new RequestError('Batched queries are not supported by this server');
             }
 
             $errors = $this->validateOperationParams($op);
 
-            if (!empty($errors)) {
-                $errors = Utils::map($errors, function(RequestError $err) {
-                    return Error::createLocatedError($err, null, null);
-                });
+            if (! empty($errors)) {
+                $errors = Utils::map(
+                    $errors,
+                    function (RequestError $err) {
+                        return Error::createLocatedError($err, null, null);
+                    }
+                );
+
                 return $promiseAdapter->createFulfilled(
                     new ExecutionResult(null, $errors)
                 );
@@ -241,13 +264,13 @@ class Helper
 
             $doc = $op->queryId ? $this->loadPersistedQuery($config, $op) : $op->query;
 
-            if (!$doc instanceof DocumentNode) {
+            if (! $doc instanceof DocumentNode) {
                 $doc = Parser::parse($doc);
             }
 
             $operationType = AST::getOperation($doc, $op->operation);
-            if ($op->isReadOnly() && $operationType !== 'query') {
-                throw new RequestError("GET supports only query operation");
+            if ($operationType !== 'query' && $op->isReadOnly()) {
+                throw new RequestError('GET supports only query operation');
             }
 
             $result = GraphQL::promiseToExecute(
@@ -261,7 +284,6 @@ class Helper
                 $config->getFieldResolver(),
                 $this->resolveValidationRules($config, $op, $doc, $operationType)
             );
-
         } catch (RequestError $e) {
             $result = $promiseAdapter->createFulfilled(
                 new ExecutionResult(null, [Error::createLocatedError($e)])
@@ -278,10 +300,13 @@ class Helper
             }
             if ($config->getErrorFormatter() || $config->getDebug()) {
                 $result->setErrorFormatter(
-                    FormattedError::prepareFormatter($config->getErrorFormatter(),
-                    $config->getDebug())
+                    FormattedError::prepareFormatter(
+                        $config->getErrorFormatter(),
+                        $config->getDebug()
+                    )
                 );
             }
+
             return $result;
         };
 
@@ -289,25 +314,23 @@ class Helper
     }
 
     /**
-     * @param ServerConfig $config
-     * @param OperationParams $op
      * @return mixed
      * @throws RequestError
      */
-    private function loadPersistedQuery(ServerConfig $config, OperationParams $op)
+    private function loadPersistedQuery(ServerConfig $config, OperationParams $operationParams)
     {
         // Load query if we got persisted query id:
         $loader = $config->getPersistentQueryLoader();
 
-        if (!$loader) {
-            throw new RequestError("Persisted queries are not supported by this server");
+        if (! $loader) {
+            throw new RequestError('Persisted queries are not supported by this server');
         }
 
-        $source = $loader($op->queryId, $op);
+        $source = $loader($operationParams->queryId, $operationParams);
 
-        if (!is_string($source) && !$source instanceof DocumentNode) {
+        if (! is_string($source) && ! $source instanceof DocumentNode) {
             throw new InvariantViolation(sprintf(
-                "Persistent query loader must return query string or instance of %s but got: %s",
+                'Persistent query loader must return query string or instance of %s but got: %s',
                 DocumentNode::class,
                 Utils::printSafe($source)
             ));
@@ -317,23 +340,24 @@ class Helper
     }
 
     /**
-     * @param ServerConfig $config
-     * @param OperationParams $params
-     * @param DocumentNode $doc
-     * @param $operationType
-     * @return array
+     * @param string $operationType
+     * @return mixed[]|null
      */
-    private function resolveValidationRules(ServerConfig $config, OperationParams $params, DocumentNode $doc, $operationType)
-    {
+    private function resolveValidationRules(
+        ServerConfig $config,
+        OperationParams $params,
+        DocumentNode $doc,
+        $operationType
+    ) {
         // Allow customizing validation rules per operation:
         $validationRules = $config->getValidationRules();
 
         if (is_callable($validationRules)) {
             $validationRules = $validationRules($params, $doc, $operationType);
 
-            if (!is_array($validationRules)) {
+            if (! is_array($validationRules)) {
                 throw new InvariantViolation(sprintf(
-                    "Expecting validation rules to be array or callable returning array, but got: %s",
+                    'Expecting validation rules to be array or callable returning array, but got: %s',
                     Utils::printSafe($validationRules)
                 ));
             }
@@ -343,10 +367,7 @@ class Helper
     }
 
     /**
-     * @param ServerConfig $config
-     * @param OperationParams $params
-     * @param DocumentNode $doc
-     * @param $operationType
+     * @param string $operationType
      * @return mixed
      */
     private function resolveRootValue(ServerConfig $config, OperationParams $params, DocumentNode $doc, $operationType)
@@ -361,14 +382,15 @@ class Helper
     }
 
     /**
-     * @param ServerConfig $config
-     * @param OperationParams $params
-     * @param DocumentNode $doc
-     * @param $operationType
+     * @param string $operationType
      * @return mixed
      */
-    private function resolveContextValue(ServerConfig $config, OperationParams $params, DocumentNode $doc, $operationType)
-    {
+    private function resolveContextValue(
+        ServerConfig $config,
+        OperationParams $params,
+        DocumentNode $doc,
+        $operationType
+    ) {
         $context = $config->getContext();
 
         if ($context instanceof \Closure) {
@@ -383,12 +405,12 @@ class Helper
      *
      * @api
      * @param Promise|ExecutionResult|ExecutionResult[] $result
-     * @param bool $exitWhenDone
+     * @param bool                                      $exitWhenDone
      */
     public function sendResponse($result, $exitWhenDone = false)
     {
         if ($result instanceof Promise) {
-            $result->then(function($actualResult) use ($exitWhenDone) {
+            $result->then(function ($actualResult) use ($exitWhenDone) {
                 $this->doSendResponse($actualResult, $exitWhenDone);
             });
         } else {
@@ -396,10 +418,6 @@ class Helper
         }
     }
 
-    /**
-     * @param $result
-     * @param $exitWhenDone
-     */
     private function doSendResponse($result, $exitWhenDone)
     {
         $httpStatus = $this->resolveHttpStatus($result);
@@ -407,9 +425,9 @@ class Helper
     }
 
     /**
-     * @param array|\JsonSerializable $jsonSerializable
-     * @param int $httpStatus
-     * @param bool $exitWhenDone
+     * @param mixed[]|\JsonSerializable $jsonSerializable
+     * @param int                       $httpStatus
+     * @param bool                      $exitWhenDone
      */
     public function emitResponse($jsonSerializable, $httpStatus, $exitWhenDone)
     {
@@ -431,37 +449,41 @@ class Helper
     }
 
     /**
-     * @param $result
+     * @param ExecutionResult|mixed[] $result
      * @return int
      */
     private function resolveHttpStatus($result)
     {
         if (is_array($result) && isset($result[0])) {
-            Utils::each($result, function ($executionResult, $index) {
-                if (!$executionResult instanceof ExecutionResult) {
-                    throw new InvariantViolation(sprintf(
-                        "Expecting every entry of batched query result to be instance of %s but entry at position %d is %s",
-                        ExecutionResult::class,
-                        $index,
-                        Utils::printSafe($executionResult)
-                    ));
+            Utils::each(
+                $result,
+                function ($executionResult, $index) {
+                    if (! $executionResult instanceof ExecutionResult) {
+                        throw new InvariantViolation(sprintf(
+                            'Expecting every entry of batched query result to be instance of %s but entry at position %d is %s',
+                            ExecutionResult::class,
+                            $index,
+                            Utils::printSafe($executionResult)
+                        ));
+                    }
                 }
-            });
+            );
             $httpStatus = 200;
         } else {
-            if (!$result instanceof ExecutionResult) {
+            if (! $result instanceof ExecutionResult) {
                 throw new InvariantViolation(sprintf(
-                    "Expecting query result to be instance of %s but got %s",
+                    'Expecting query result to be instance of %s but got %s',
                     ExecutionResult::class,
                     Utils::printSafe($result)
                 ));
             }
-            if ($result->data === null && !empty($result->errors)) {
+            if ($result->data === null && ! empty($result->errors)) {
                 $httpStatus = 400;
             } else {
                 $httpStatus = 200;
             }
         }
+
         return $httpStatus;
     }
 
@@ -469,8 +491,7 @@ class Helper
      * Converts PSR-7 request to OperationParams[]
      *
      * @api
-     * @param ServerRequestInterface $request
-     * @return array|Helper
+     * @return OperationParams[]|OperationParams
      * @throws RequestError
      */
     public function parsePsrRequest(ServerRequestInterface $request)
@@ -480,32 +501,32 @@ class Helper
         } else {
             $contentType = $request->getHeader('content-type');
 
-            if (!isset($contentType[0])) {
+            if (! isset($contentType[0])) {
                 throw new RequestError('Missing "Content-Type" header');
             }
 
             if (stripos($contentType[0], 'application/graphql') !== false) {
                 $bodyParams = ['query' => $request->getBody()->getContents()];
-            } else if (stripos($contentType[0], 'application/json') !== false) {
+            } elseif (stripos($contentType[0], 'application/json') !== false) {
                 $bodyParams = $request->getParsedBody();
 
-                if (null === $bodyParams) {
+                if ($bodyParams === null) {
                     throw new InvariantViolation(
-                        "PSR-7 request is expected to provide parsed body for \"application/json\" requests but got null"
+                        'PSR-7 request is expected to provide parsed body for "application/json" requests but got null'
                     );
                 }
 
-                if (!is_array($bodyParams)) {
+                if (! is_array($bodyParams)) {
                     throw new RequestError(
-                        "GraphQL Server expects JSON object or array, but got " .
+                        'GraphQL Server expects JSON object or array, but got ' .
                         Utils::printSafeJson($bodyParams)
                     );
                 }
             } else {
                 $bodyParams = $request->getParsedBody();
 
-                if (!is_array($bodyParams)) {
-                    throw new RequestError("Unexpected content type: " . Utils::printSafeJson($contentType[0]));
+                if (! is_array($bodyParams)) {
+                    throw new RequestError('Unexpected content type: ' . Utils::printSafeJson($contentType[0]));
                 }
             }
         }
@@ -522,19 +543,17 @@ class Helper
      *
      * @api
      * @param Promise|ExecutionResult|ExecutionResult[] $result
-     * @param ResponseInterface $response
-     * @param StreamInterface $writableBodyStream
      * @return Promise|ResponseInterface
      */
     public function toPsrResponse($result, ResponseInterface $response, StreamInterface $writableBodyStream)
     {
         if ($result instanceof Promise) {
-            return $result->then(function($actualResult) use ($response, $writableBodyStream) {
+            return $result->then(function ($actualResult) use ($response, $writableBodyStream) {
                 return $this->doConvertToPsrResponse($actualResult, $response, $writableBodyStream);
             });
-        } else {
-            return $this->doConvertToPsrResponse($result, $response, $writableBodyStream);
         }
+
+        return $this->doConvertToPsrResponse($result, $response, $writableBodyStream);
     }
 
     private function doConvertToPsrResponse($result, ResponseInterface $response, StreamInterface $writableBodyStream)
diff --git a/src/Server/OperationParams.php b/src/Server/OperationParams.php
index 85782fe..f592a95 100644
--- a/src/Server/OperationParams.php
+++ b/src/Server/OperationParams.php
@@ -1,6 +1,15 @@
 <?php
+
+declare(strict_types=1);
+
 namespace GraphQL\Server;
 
+use const CASE_LOWER;
+use function array_change_key_case;
+use function is_string;
+use function json_decode;
+use function json_last_error;
+
 /**
  * Structure representing parsed HTTP parameters for GraphQL operation
  */
@@ -33,33 +42,29 @@ class OperationParams
 
     /**
      * @api
-     * @var array
+     * @var mixed[]|null
      */
     public $variables;
 
-    /**
-     * @var array
-     */
+    /** @var mixed[] */
     private $originalInput;
 
-    /**
-     * @var bool
-     */
+    /** @var bool */
     private $readOnly;
 
     /**
      * Creates an instance from given array
      *
      * @api
-     * @param array $params
-     * @param bool $readonly
+     * @param mixed[] $params
+     * @param bool    $readonly
      * @return OperationParams
      */
     public static function create(array $params, $readonly = false)
     {
         $instance = new static();
 
-        $params = array_change_key_case($params, CASE_LOWER);
+        $params                  = array_change_key_case($params, CASE_LOWER);
         $instance->originalInput = $params;
 
         $params += [
@@ -68,25 +73,25 @@ class OperationParams
             'documentid' => null, // alias to queryid
             'id' => null, // alias to queryid
             'operationname' => null,
-            'variables' => null
+            'variables' => null,
         ];
 
-        if ($params['variables'] === "") {
+        if ($params['variables'] === '') {
             $params['variables'] = null;
         }
 
         if (is_string($params['variables'])) {
             $tmp = json_decode($params['variables'], true);
-            if (!json_last_error()) {
+            if (! json_last_error()) {
                 $params['variables'] = $tmp;
             }
         }
 
-        $instance->query = $params['query'];
-        $instance->queryId = $params['queryid'] ?: $params['documentid'] ?: $params['id'];
+        $instance->query     = $params['query'];
+        $instance->queryId   = $params['queryid'] ?: $params['documentid'] ?: $params['id'];
         $instance->operation = $params['operationname'];
         $instance->variables = $params['variables'];
-        $instance->readOnly = (bool) $readonly;
+        $instance->readOnly  = (bool) $readonly;
 
         return $instance;
     }
@@ -98,7 +103,7 @@ class OperationParams
      */
     public function getOriginalInput($key)
     {
-        return isset($this->originalInput[$key]) ? $this->originalInput[$key] : null;
+        return $this->originalInput[$key] ?? null;
     }
 
     /**
diff --git a/src/Server/RequestError.php b/src/Server/RequestError.php
index d07f527..cde74f4 100644
--- a/src/Server/RequestError.php
+++ b/src/Server/RequestError.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 namespace GraphQL\Server;
 
 use GraphQL\Error\ClientAware;
diff --git a/src/Server/ServerConfig.php b/src/Server/ServerConfig.php
index 6277ba8..bf612aa 100644
--- a/src/Server/ServerConfig.php
+++ b/src/Server/ServerConfig.php
@@ -1,10 +1,19 @@
 <?php
+
+declare(strict_types=1);
+
 namespace GraphQL\Server;
 
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Executor\Promise\PromiseAdapter;
 use GraphQL\Type\Schema;
 use GraphQL\Utils\Utils;
+use GraphQL\Validator\Rules\AbstractValidationRule;
+use function is_array;
+use function is_callable;
+use function method_exists;
+use function sprintf;
+use function ucfirst;
 
 /**
  * Server configuration class.
@@ -26,7 +35,7 @@ class ServerConfig
      * (or just returns empty config when array is not passed).
      *
      * @api
-     * @param array $config
+     * @param mixed[] $config
      * @return ServerConfig
      */
     public static function create(array $config = [])
@@ -34,99 +43,80 @@ class ServerConfig
         $instance = new static();
         foreach ($config as $key => $value) {
             $method = 'set' . ucfirst($key);
-            if (!method_exists($instance, $method)) {
-                throw new InvariantViolation("Unknown server config option \"$key\"");
+            if (! method_exists($instance, $method)) {
+                throw new InvariantViolation(sprintf('Unknown server config option "%s"', $key));
             }
             $instance->$method($value);
         }
+
         return $instance;
     }
 
-    /**
-     * @var Schema
-     */
+    /** @var Schema */
     private $schema;
 
-    /**
-     * @var mixed|\Closure
-     */
+    /** @var mixed|\Closure */
     private $context;
 
-    /**
-     * @var mixed|\Closure
-     */
+    /** @var mixed|\Closure */
     private $rootValue;
 
-    /**
-     * @var callable|null
-     */
+    /** @var callable|null */
     private $errorFormatter;
 
-    /**
-     * @var callable|null
-     */
+    /** @var callable|null */
     private $errorsHandler;
 
-    /**
-     * @var bool
-     */
+    /** @var bool */
     private $debug = false;
 
-    /**
-     * @var bool
-     */
+    /** @var bool */
     private $queryBatching = false;
 
-    /**
-     * @var array|callable
-     */
+    /** @var AbstractValidationRule[]|callable */
     private $validationRules;
 
-    /**
-     * @var callable
-     */
+    /** @var callable */
     private $fieldResolver;
 
-    /**
-     * @var PromiseAdapter
-     */
+    /** @var PromiseAdapter */
     private $promiseAdapter;
 
-    /**
-     * @var callable
-     */
+    /** @var callable */
     private $persistentQueryLoader;
 
     /**
      * @api
-     * @param Schema $schema
-     * @return $this
+     * @return self
      */
     public function setSchema(Schema $schema)
     {
         $this->schema = $schema;
+
         return $this;
     }
 
     /**
      * @api
      * @param mixed|\Closure $context
-     * @return $this
+     * @return self
      */
     public function setContext($context)
     {
         $this->context = $context;
+
         return $this;
     }
 
     /**
      * @api
      * @param mixed|\Closure $rootValue
-     * @return $this
+     * @return self
      */
     public function setRootValue($rootValue)
     {
         $this->rootValue = $rootValue;
+
         return $this;
     }
 
@@ -134,12 +124,12 @@ class ServerConfig
      * Expects function(Throwable $e) : array
      *
      * @api
-     * @param callable $errorFormatter
-     * @return $this
+     * @return self
      */
     public function setErrorFormatter(callable $errorFormatter)
     {
         $this->errorFormatter = $errorFormatter;
+
         return $this;
     }
 
@@ -147,12 +137,12 @@ class ServerConfig
      * Expects function(array $errors, callable $formatter) : array
      *
      * @api
-     * @param callable $handler
-     * @return $this
+     * @return self
      */
     public function setErrorsHandler(callable $handler)
     {
         $this->errorsHandler = $handler;
+
         return $this;
     }
 
@@ -160,12 +150,12 @@ class ServerConfig
      * Set validation rules for this server.
      *
      * @api
-     * @param array|callable
-     * @return $this
+     * @param AbstractValidationRule[]|callable $validationRules
+     * @return self
      */
     public function setValidationRules($validationRules)
     {
-        if (!is_callable($validationRules) && !is_array($validationRules) && $validationRules !== null) {
+        if (! is_callable($validationRules) && ! is_array($validationRules) && $validationRules !== null) {
             throw new InvariantViolation(
                 'Server config expects array of validation rules or callable returning such array, but got ' .
                 Utils::printSafe($validationRules)
@@ -173,17 +163,18 @@ class ServerConfig
         }
 
         $this->validationRules = $validationRules;
+
         return $this;
     }
 
     /**
      * @api
-     * @param callable $fieldResolver
-     * @return $this
+     * @return self
      */
     public function setFieldResolver(callable $fieldResolver)
     {
         $this->fieldResolver = $fieldResolver;
+
         return $this;
     }
 
@@ -193,12 +184,12 @@ class ServerConfig
      * This function must return query string or valid DocumentNode.
      *
      * @api
-     * @param callable $persistentQueryLoader
-     * @return $this
+     * @return self
      */
     public function setPersistentQueryLoader(callable $persistentQueryLoader)
     {
         $this->persistentQueryLoader = $persistentQueryLoader;
+
         return $this;
     }
 
@@ -207,11 +198,12 @@ class ServerConfig
      *
      * @api
      * @param bool|int $set
-     * @return $this
+     * @return self
      */
     public function setDebug($set = true)
     {
         $this->debug = $set;
+
         return $this;
     }
 
@@ -220,22 +212,23 @@ class ServerConfig
      *
      * @api
      * @param bool $enableBatching
-     * @return $this
+     * @return self
      */
     public function setQueryBatching($enableBatching)
     {
         $this->queryBatching = (bool) $enableBatching;
+
         return $this;
     }
 
     /**
      * @api
-     * @param PromiseAdapter $promiseAdapter
-     * @return $this
+     * @return self
      */
     public function setPromiseAdapter(PromiseAdapter $promiseAdapter)
     {
         $this->promiseAdapter = $promiseAdapter;
+
         return $this;
     }
 
@@ -288,7 +281,7 @@ class ServerConfig
     }
 
     /**
-     * @return array|callable
+     * @return AbstractValidationRule[]|callable
      */
     public function getValidationRules()
     {
diff --git a/src/Server/StandardServer.php b/src/Server/StandardServer.php
index 9684205..c631a83 100644
--- a/src/Server/StandardServer.php
+++ b/src/Server/StandardServer.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 namespace GraphQL\Server;
 
 use GraphQL\Error\FormattedError;
@@ -9,6 +12,7 @@ use GraphQL\Utils;
 use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\ServerRequestInterface;
 use Psr\Http\Message\StreamInterface;
+use function is_array;
 
 /**
  * GraphQL server compatible with both: [express-graphql](https://github.com/graphql/express-graphql)
@@ -34,14 +38,10 @@ use Psr\Http\Message\StreamInterface;
  */
 class StandardServer
 {
-    /**
-     * @var ServerConfig
-     */
+    /** @var ServerConfig */
     private $config;
 
-    /**
-     * @var Helper
-     */
+    /** @var Helper */
     private $helper;
 
     /**
@@ -51,17 +51,15 @@ class StandardServer
      *
      * @api
      * @param \Throwable $error
-     * @param bool $debug
-     * @param bool $exitWhenDone
+     * @param bool       $debug
+     * @param bool       $exitWhenDone
      */
     public static function send500Error($error, $debug = false, $exitWhenDone = false)
     {
         $response = [
-            'errors' => [
-                FormattedError::createFromException($error, $debug)
-            ]
+            'errors' => [FormattedError::createFromException($error, $debug)],
         ];
-        $helper = new Helper();
+        $helper   = new Helper();
         $helper->emitResponse($response, 500, $exitWhenDone);
     }
 
@@ -69,15 +67,15 @@ class StandardServer
      * Creates new instance of a standard GraphQL HTTP server
      *
      * @api
-     * @param ServerConfig|array $config
+     * @param ServerConfig|mixed[] $config
      */
     public function __construct($config)
     {
         if (is_array($config)) {
             $config = ServerConfig::create($config);
         }
-        if (!$config instanceof ServerConfig) {
-            throw new InvariantViolation("Expecting valid server config, but got " . Utils::printSafe($config));
+        if (! $config instanceof ServerConfig) {
+            throw new InvariantViolation('Expecting valid server config, but got ' . Utils::printSafe($config));
         }
         $this->config = $config;
         $this->helper = new Helper();
@@ -95,7 +93,7 @@ class StandardServer
      *
      * @api
      * @param OperationParams|OperationParams[] $parsedBody
-     * @param bool $exitWhenDone
+     * @param bool                              $exitWhenDone
      */
     public function handleRequest($parsedBody = null, $exitWhenDone = false)
     {
@@ -120,15 +118,15 @@ class StandardServer
      */
     public function executeRequest($parsedBody = null)
     {
-        if (null === $parsedBody) {
+        if ($parsedBody === null) {
             $parsedBody = $this->helper->parseHttpRequest();
         }
 
         if (is_array($parsedBody)) {
             return $this->helper->executeBatch($this->config, $parsedBody);
-        } else {
-            return $this->helper->executeOperation($this->config, $parsedBody);
         }
+
+        return $this->helper->executeOperation($this->config, $parsedBody);
     }
 
     /**
@@ -138,17 +136,13 @@ class StandardServer
      * (e.g. using specific JsonResponse instance of some framework).
      *
      * @api
-     * @param ServerRequestInterface $request
-     * @param ResponseInterface $response
-     * @param StreamInterface $writableBodyStream
      * @return ResponseInterface|Promise
      */
     public function processPsrRequest(
         ServerRequestInterface $request,
         ResponseInterface $response,
         StreamInterface $writableBodyStream
-    )
-    {
+    ) {
         $result = $this->executePsrRequest($request);
         return $this->helper->toPsrResponse($result, $response, $writableBodyStream);
     }
@@ -158,7 +152,6 @@ class StandardServer
      * (or promise when promise adapter is different from SyncPromiseAdapter)
      *
      * @api
-     * @param ServerRequestInterface $request
      * @return ExecutionResult|ExecutionResult[]|Promise
      */
     public function executePsrRequest(ServerRequestInterface $request)