diff --git a/src/Error/Warning.php b/src/Error/Warning.php
index 25828b1..6728941 100644
--- a/src/Error/Warning.php
+++ b/src/Error/Warning.php
@@ -96,7 +96,7 @@ final class Warning
 
     public static function warnOnce(string $errorMessage, int $warningId, ?int $messageLevel = null) : void
     {
-        if (self::$warningHandler) {
+        if (self::$warningHandler !== null) {
             $fn = self::$warningHandler;
             $fn($errorMessage, $warningId);
         } elseif ((self::$enableWarnings & $warningId) > 0 && ! isset(self::$warned[$warningId])) {
@@ -107,7 +107,7 @@ final class Warning
 
     public static function warn(string $errorMessage, int $warningId, ?int $messageLevel = null) : void
     {
-        if (self::$warningHandler) {
+        if (self::$warningHandler !== null) {
             $fn = self::$warningHandler;
             $fn($errorMessage, $warningId);
         } elseif ((self::$enableWarnings & $warningId) > 0) {
diff --git a/src/Executor/Promise/Adapter/SyncPromise.php b/src/Executor/Promise/Adapter/SyncPromise.php
index 26aecd6..8d490ed 100644
--- a/src/Executor/Promise/Adapter/SyncPromise.php
+++ b/src/Executor/Promise/Adapter/SyncPromise.php
@@ -38,16 +38,16 @@ class SyncPromise
      */
     private $waiting = [];
 
-    public static function runQueue()
+    public static function runQueue() : void
     {
         $q = self::$queue;
-        while ($q && ! $q->isEmpty()) {
+        while ($q !== null && ! $q->isEmpty()) {
             $task = $q->dequeue();
             $task();
         }
     }
 
-    public function resolve($value)
+    public function resolve($value) : self
     {
         switch ($this->state) {
             case self::PENDING:
@@ -83,7 +83,7 @@ class SyncPromise
         return $this;
     }
 
-    public function reject($reason)
+    public function reject($reason) : self
     {
         if (! $reason instanceof Exception && ! $reason instanceof Throwable) {
             throw new Exception('SyncPromise::reject() has to be called with an instance of \Throwable');
@@ -107,7 +107,7 @@ class SyncPromise
         return $this;
     }
 
-    private function enqueueWaitingPromises()
+    private function enqueueWaitingPromises() : void
     {
         Utils::invariant(
             $this->state !== self::PENDING,
@@ -116,7 +116,7 @@ class SyncPromise
 
         foreach ($this->waiting as $descriptor) {
             self::getQueue()->enqueue(function () use ($descriptor) {
-                /** @var $promise self */
+                /** @var self $promise */
                 [$promise, $onFulfilled, $onRejected] = $descriptor;
 
                 if ($this->state === self::FULFILLED) {
@@ -145,17 +145,17 @@ class SyncPromise
         $this->waiting = [];
     }
 
-    public static function getQueue()
+    public static function getQueue() : SplQueue
     {
         return self::$queue ?: self::$queue = new SplQueue();
     }
 
     public function then(?callable $onFulfilled = null, ?callable $onRejected = null)
     {
-        if ($this->state === self::REJECTED && ! $onRejected) {
+        if ($this->state === self::REJECTED && $onRejected === null) {
             return $this;
         }
-        if ($this->state === self::FULFILLED && ! $onFulfilled) {
+        if ($this->state === self::FULFILLED && $onFulfilled === null) {
             return $this;
         }
         $tmp             = new self();
diff --git a/src/Executor/Values.php b/src/Executor/Values.php
index 2f74e8c..82e26c8 100644
--- a/src/Executor/Values.php
+++ b/src/Executor/Values.php
@@ -92,7 +92,7 @@ class Values
                             ),
                             [$varDefNode]
                         );
-                    } elseif ($varDefNode->defaultValue) {
+                    } elseif ($varDefNode->defaultValue !== null) {
                         $coercedValues[$varName] = AST::valueFromAST($varDefNode->defaultValue, $varType);
                     }
                 }
@@ -196,7 +196,7 @@ class Values
             $argType           = $argumentDefinition->getType();
             $argumentValueNode = $argumentValueMap[$name] ?? null;
 
-            if (! $argumentValueNode) {
+            if ($argumentValueNode === null) {
                 if ($argumentDefinition->defaultValueExists()) {
                     $coercedValues[$name] = $argumentDefinition->defaultValue;
                 } elseif ($argType instanceof NonNull) {
@@ -209,7 +209,7 @@ class Values
             } elseif ($argumentValueNode instanceof VariableNode) {
                 $variableName = $argumentValueNode->name->value;
 
-                if ($variableValues && array_key_exists($variableName, $variableValues)) {
+                if ($variableValues !== null && array_key_exists($variableName, $variableValues)) {
                     // Note: this does not check that this variable value is correct.
                     // This assumes that this query has been validated and the variable
                     // usage here is of the correct type.
diff --git a/src/Experimental/Executor/Collector.php b/src/Experimental/Executor/Collector.php
index 8639335..843db9b 100644
--- a/src/Experimental/Executor/Collector.php
+++ b/src/Experimental/Executor/Collector.php
@@ -199,7 +199,7 @@ class Collector
             if ($selection instanceof FieldNode) {
                 /** @var FieldNode $selection */
 
-                $resultName = $selection->alias ? $selection->alias->value : $selection->name->value;
+                $resultName = $selection->alias === null ? $selection->name->value : $selection->alias->value;
 
                 if (! isset($this->fields[$resultName])) {
                     $this->fields[$resultName] = [];
diff --git a/src/Experimental/Executor/CoroutineExecutor.php b/src/Experimental/Executor/CoroutineExecutor.php
index 91091b4..d90035b 100644
--- a/src/Experimental/Executor/CoroutineExecutor.php
+++ b/src/Experimental/Executor/CoroutineExecutor.php
@@ -498,7 +498,7 @@ class CoroutineExecutor implements Runtime, ExecutorImplementation
 
         if ($type !== $this->schema->getType($type->name)) {
             $hint = '';
-            if ($this->schema->getConfig()->typeLoader) {
+            if ($this->schema->getConfig()->typeLoader !== null) {
                 $hint = sprintf(
                     'Make sure that type loader returns the same instance as defined in %s.%s',
                     $ctx->type,
@@ -646,7 +646,7 @@ class CoroutineExecutor implements Runtime, ExecutorImplementation
         } else {
             if ($type !== $this->schema->getType($type->name)) {
                 $hint = '';
-                if ($this->schema->getConfig()->typeLoader) {
+                if ($this->schema->getConfig()->typeLoader !== null) {
                     $hint = sprintf(
                         'Make sure that type loader returns the same instance as defined in %s.%s',
                         $ctx->type,
@@ -904,7 +904,7 @@ class CoroutineExecutor implements Runtime, ExecutorImplementation
             return $this->schema->getType($value['__typename']);
         }
 
-        if ($abstractType instanceof InterfaceType && $this->schema->getConfig()->typeLoader) {
+        if ($abstractType instanceof InterfaceType && $this->schema->getConfig()->typeLoader !== null) {
             Warning::warnOnce(
                 sprintf(
                     'GraphQL Interface Type `%s` returned `null` from its `resolveType` function ' .
diff --git a/src/Language/AST/Location.php b/src/Language/AST/Location.php
index 72135fc..6f2e6d4 100644
--- a/src/Language/AST/Location.php
+++ b/src/Language/AST/Location.php
@@ -69,7 +69,7 @@ class Location
         $this->endToken   = $endToken;
         $this->source     = $source;
 
-        if (! $startToken || ! $endToken) {
+        if ($startToken === null || $endToken === null) {
             return;
         }
 
diff --git a/src/Language/AST/Node.php b/src/Language/AST/Node.php
index 2205d6d..0e192af 100644
--- a/src/Language/AST/Node.php
+++ b/src/Language/AST/Node.php
@@ -106,7 +106,7 @@ abstract class Node
 
         $tmp = (array) $this;
 
-        if ($this->loc) {
+        if ($this->loc !== null) {
             $tmp['loc'] = [
                 'start' => $this->loc->start,
                 'end'   => $this->loc->end,
@@ -125,7 +125,7 @@ abstract class Node
             'kind' => $node->kind,
         ];
 
-        if ($node->loc) {
+        if ($node->loc !== null) {
             $result['loc'] = [
                 'start' => $node->loc->start,
                 'end'   => $node->loc->end,
diff --git a/src/Language/Lexer.php b/src/Language/Lexer.php
index 3aad4dd..d2c1d5f 100644
--- a/src/Language/Lexer.php
+++ b/src/Language/Lexer.php
@@ -314,11 +314,11 @@ class Lexer
         $start         = $this->position;
         [$char, $code] = $this->readChar();
 
-        while ($code && (
+        while ($code !== null && (
                 $code === 95 || // _
-                $code >= 48 && $code <= 57 || // 0-9
-                $code >= 65 && $code <= 90 || // A-Z
-                $code >= 97 && $code <= 122 // a-z
+                ($code >= 48 && $code <= 57) || // 0-9
+                ($code >= 65 && $code <= 90) || // A-Z
+                ($code >= 97 && $code <= 122) // a-z
             )) {
             $value        .= $char;
             [$char, $code] = $this->moveStringCursor(1, 1)->readChar();
@@ -695,7 +695,7 @@ class Lexer
         do {
             [$char, $code, $bytes] = $this->moveStringCursor(1, $bytes)->readChar();
             $value                .= $char;
-        } while ($code &&
+        } while ($code !== null &&
         // SourceCharacter but not LineTerminator
         ($code > 0x001F || $code === 0x0009)
         );
diff --git a/src/Language/Parser.php b/src/Language/Parser.php
index f5c9280..a0c123f 100644
--- a/src/Language/Parser.php
+++ b/src/Language/Parser.php
@@ -1655,9 +1655,7 @@ class Parser
         $name       = $this->parseName();
         $directives = $this->parseDirectives(true);
         $types      = $this->parseUnionMemberTypes();
-        if (count($directives) === 0 &&
-            ! $types
-        ) {
+        if (count($directives) === 0 && count($types) === 0) {
             throw $this->unexpected();
         }
 
diff --git a/src/Language/Token.php b/src/Language/Token.php
index 831b3ad..eb0bbdf 100644
--- a/src/Language/Token.php
+++ b/src/Language/Token.php
@@ -11,28 +11,28 @@ namespace GraphQL\Language;
 class Token
 {
     // Each kind of token.
-    const SOF          = '<SOF>';
-    const EOF          = '<EOF>';
-    const BANG         = '!';
-    const DOLLAR       = '$';
-    const AMP          = '&';
-    const PAREN_L      = '(';
-    const PAREN_R      = ')';
-    const SPREAD       = '...';
-    const COLON        = ':';
-    const EQUALS       = '=';
-    const AT           = '@';
-    const BRACKET_L    = '[';
-    const BRACKET_R    = ']';
-    const BRACE_L      = '{';
-    const PIPE         = '|';
-    const BRACE_R      = '}';
-    const NAME         = 'Name';
-    const INT          = 'Int';
-    const FLOAT        = 'Float';
-    const STRING       = 'String';
-    const BLOCK_STRING = 'BlockString';
-    const COMMENT      = 'Comment';
+    public const SOF          = '<SOF>';
+    public const EOF          = '<EOF>';
+    public const BANG         = '!';
+    public const DOLLAR       = '$';
+    public const AMP          = '&';
+    public const PAREN_L      = '(';
+    public const PAREN_R      = ')';
+    public const SPREAD       = '...';
+    public const COLON        = ':';
+    public const EQUALS       = '=';
+    public const AT           = '@';
+    public const BRACKET_L    = '[';
+    public const BRACKET_R    = ']';
+    public const BRACE_L      = '{';
+    public const PIPE         = '|';
+    public const BRACE_R      = '}';
+    public const NAME         = 'Name';
+    public const INT          = 'Int';
+    public const FLOAT        = 'Float';
+    public const STRING       = 'String';
+    public const BLOCK_STRING = 'BlockString';
+    public const COMMENT      = 'Comment';
 
     /**
      * The kind of Token (see one of constants above).
@@ -104,18 +104,15 @@ class Token
         $this->value  = $value;
     }
 
-    /**
-     * @return string
-     */
-    public function getDescription()
+    public function getDescription() : string
     {
-        return $this->kind . ($this->value ? ' "' . $this->value . '"' : '');
+        return $this->kind . ($this->value === null ? '' : ' "' . $this->value . '"');
     }
 
     /**
      * @return (string|int|null)[]
      */
-    public function toArray()
+    public function toArray() : array
     {
         return [
             'kind'   => $this->kind,
diff --git a/src/Server/OperationParams.php b/src/Server/OperationParams.php
index 3ab12de..a976ec0 100644
--- a/src/Server/OperationParams.php
+++ b/src/Server/OperationParams.php
@@ -9,6 +9,7 @@ use function is_string;
 use function json_decode;
 use function json_last_error;
 use const CASE_LOWER;
+use const JSON_ERROR_NONE;
 
 /**
  * Structure representing parsed HTTP parameters for GraphQL operation
@@ -93,7 +94,7 @@ class OperationParams
             }
 
             $tmp = json_decode($params[$param], true);
-            if (json_last_error()) {
+            if (json_last_error() !== JSON_ERROR_NONE) {
                 continue;
             }
 
diff --git a/src/Type/Definition/Directive.php b/src/Type/Definition/Directive.php
index 0ffe496..3ad4b8a 100644
--- a/src/Type/Definition/Directive.php
+++ b/src/Type/Definition/Directive.php
@@ -9,6 +9,7 @@ use GraphQL\Language\DirectiveLocation;
 use GraphQL\Utils\Utils;
 use function array_key_exists;
 use function array_keys;
+use function count;
 use function in_array;
 use function is_array;
 
@@ -16,11 +17,11 @@ class Directive
 {
     public const DEFAULT_DEPRECATION_REASON = 'No longer supported';
 
-    const INCLUDE_NAME         = 'include';
-    const IF_ARGUMENT_NAME     = 'if';
-    const SKIP_NAME            = 'skip';
-    const DEPRECATED_NAME      = 'deprecated';
-    const REASON_ARGUMENT_NAME = 'reason';
+    public const INCLUDE_NAME         = 'include';
+    public const IF_ARGUMENT_NAME     = 'if';
+    public const SKIP_NAME            = 'skip';
+    public const DEPRECATED_NAME      = 'deprecated';
+    public const REASON_ARGUMENT_NAME = 'reason';
 
     /** @var Directive[] */
     public static $internalDirectives;
@@ -84,9 +85,9 @@ class Directive
     /**
      * @return Directive[]
      */
-    public static function getInternalDirectives()
+    public static function getInternalDirectives() : array
     {
-        if (! self::$internalDirectives) {
+        if (count(self::$internalDirectives) === 0) {
             self::$internalDirectives = [
                 'include'    => new self([
                     'name'        => self::INCLUDE_NAME,
diff --git a/src/Type/Definition/ObjectType.php b/src/Type/Definition/ObjectType.php
index f509293..532a9b0 100644
--- a/src/Type/Definition/ObjectType.php
+++ b/src/Type/Definition/ObjectType.php
@@ -168,7 +168,7 @@ class ObjectType extends Type implements OutputType, CompositeType, NullableType
 
     private function getInterfaceMap()
     {
-        if (! $this->interfaceMap) {
+        if ($this->interfaceMap === null) {
             $this->interfaceMap = [];
             foreach ($this->getInterfaces() as $interface) {
                 $this->interfaceMap[$interface->name] = $interface;
diff --git a/src/Type/SchemaConfig.php b/src/Type/SchemaConfig.php
index 9f33626..1b21775 100644
--- a/src/Type/SchemaConfig.php
+++ b/src/Type/SchemaConfig.php
@@ -42,7 +42,7 @@ class SchemaConfig
     /** @var Directive[] */
     public $directives;
 
-    /** @var callable */
+    /** @var callable|null */
     public $typeLoader;
 
     /** @var SchemaDefinitionNode */