diff --git a/Account/Account.php b/Account/Account.php
index 5ed5beeef..6c9ef5834 100644
--- a/Account/Account.php
+++ b/Account/Account.php
@@ -320,15 +320,15 @@ class Account implements ArrayableInterface, \JsonSerializable
      */
     public function hasPermission(int $permission, int $unit = null, string $app = null, int $module = null, int $type = null, $element = null, $component = null) : bool
     {
-        $app = isset($app) ? strtolower($app) : $app;
+        $app = $app !== null ? strtolower($app) : $app;
 
         foreach ($this->permissions as $p) {
-            if (($p->getUnit() === $unit || $p->getUnit() === null || !isset($unit))
-                && ($p->getApp() === $app || $p->getApp() === null || !isset($app))
-                && ($p->getModule() === $module || $p->getModule() === null || !isset($module))
-                && ($p->getType() === $type || $p->getType() === null || !isset($type))
-                && ($p->getElement() === $element || $p->getElement() === null || !isset($element))
-                && ($p->getComponent() === $component || $p->getComponent() === null || !isset($component))
+            if (($p->getUnit() === $unit || $p->getUnit() === null || $unit === null)
+                && ($p->getApp() === $app || $p->getApp() === null || $app === null)
+                && ($p->getModule() === $module || $p->getModule() === null || $module === null)
+                && ($p->getType() === $type || $p->getType() === null || $type === null)
+                && ($p->getElement() === $element || $p->getElement() === null || $element === null)
+                && ($p->getComponent() === $component || $p->getComponent() === null || $component === null)
                 && ($p->getPermission() | $permission) === $p->getPermission()
             ) {
                 return true;
diff --git a/DataStorage/Database/Connection/ConnectionAbstract.php b/DataStorage/Database/Connection/ConnectionAbstract.php
index 64a79fd3d..e9f3041c0 100644
--- a/DataStorage/Database/Connection/ConnectionAbstract.php
+++ b/DataStorage/Database/Connection/ConnectionAbstract.php
@@ -161,7 +161,7 @@ abstract class ConnectionAbstract implements ConnectionInterface
      */
     public function getGrammar() : Grammar
     {
-        if (!isset($this->grammar)) {
+        if ($this->grammar === null) {
             $this->grammar = new Grammar();
         }
 
@@ -173,7 +173,7 @@ abstract class ConnectionAbstract implements ConnectionInterface
      */
     public function getSchemaGrammar() : SchemaGrammar
     {
-        if (!isset($this->schemaGrammar)) {
+        if ($this->schemaGrammar === null) {
             $this->schemaGrammar = new SchemaGrammar();
         }
 
diff --git a/DataStorage/Database/Connection/SQLiteConnection.php b/DataStorage/Database/Connection/SQLiteConnection.php
index 3b32191bb..dd4df8a5d 100644
--- a/DataStorage/Database/Connection/SQLiteConnection.php
+++ b/DataStorage/Database/Connection/SQLiteConnection.php
@@ -55,7 +55,7 @@ class SqliteConnection extends ConnectionAbstract
     {
         $this->close();
 
-        $this->dbdata = isset($dbdata) ? $dbdata : $this->dbdata;
+        $this->dbdata = $dbdata !== null ? $dbdata : $this->dbdata;
         $this->prefix = $dbdata['prefix'];
 
         try {
diff --git a/DataStorage/Database/Query/Builder.php b/DataStorage/Database/Query/Builder.php
index 89efe010c..eb813626d 100644
--- a/DataStorage/Database/Query/Builder.php
+++ b/DataStorage/Database/Query/Builder.php
@@ -486,7 +486,7 @@ class Builder extends BuilderAbstract
      */
     public function where($columns, $operator = null, $values = null, $boolean = 'and') : Builder
     {
-        if (isset($operator) && !is_array($operator) && !in_array(strtolower($operator), self::OPERATORS)) {
+        if ($operator !== null && !is_array($operator) && !in_array(strtolower($operator), self::OPERATORS)) {
             throw new \InvalidArgumentException('Unknown operator.');
         }
 
diff --git a/Localization/L11nManager.php b/Localization/L11nManager.php
index d5cb76f62..99ae9a18f 100644
--- a/Localization/L11nManager.php
+++ b/Localization/L11nManager.php
@@ -126,7 +126,7 @@ class L11nManager
      */
     public function getModuleLanguage(string $language, string $module = null) : array
     {
-        if (!isset($module) && isset($this->language[$language])) {
+        if ($module === null && isset($this->language[$language])) {
             return $this->language[$language];
         } elseif (isset($this->language[$language], $this->language[$language][$module])) {
             return $this->language[$language][$module];
diff --git a/Math/Statistic/MeasureOfDispersion.php b/Math/Statistic/MeasureOfDispersion.php
index 5eb1d801a..36c6f8d75 100644
--- a/Math/Statistic/MeasureOfDispersion.php
+++ b/Math/Statistic/MeasureOfDispersion.php
@@ -64,7 +64,7 @@ class MeasureOfDispersion
      */
     public static function empiricalVariationCoefficient(array $values, float $mean = null) : float
     {
-        $mean = isset($mean) ? $mean : Average::arithmeticMean($values);
+        $mean = $mean !== null ? $mean : Average::arithmeticMean($values);
 
         if ($mean === 0) {
             throw new ZeroDevisionException();
@@ -89,7 +89,7 @@ class MeasureOfDispersion
      */
     public static function standardDeviation(array $values, float $mean = null) : float
     {
-        $mean = isset($mean) ? $mean : Average::arithmeticMean($values);
+        $mean = $mean !== null ? $mean : Average::arithmeticMean($values);
         $sum  = 0.0;
 
         foreach ($values as $value) {
@@ -156,7 +156,7 @@ class MeasureOfDispersion
             throw new ZeroDevisionException();
         }
 
-        $mean = $hasProbability ? Average::weightedAverage($values, $probabilities) : (isset($mean) ? $mean : Average::arithmeticMean($values));
+        $mean = $hasProbability ? Average::weightedAverage($values, $probabilities) : ($mean !== null ? $mean : Average::arithmeticMean($values));
         $sum  = 0;
 
         foreach ($values as $key => $value) {
@@ -196,8 +196,8 @@ class MeasureOfDispersion
             throw new InvalidDimensionException($count . 'x' . count($y));
         }
 
-        $xMean = isset($meanX) ? $meanX : Average::arithmeticMean($x);
-        $yMean = isset($meanY) ? $meanY : Average::arithmeticMean($y);
+        $xMean = $meanX !== null ? $meanX : Average::arithmeticMean($x);
+        $yMean = $meanY !== null ? $meanY : Average::arithmeticMean($y);
 
         $sum = 0.0;
 
@@ -234,7 +234,7 @@ class MeasureOfDispersion
      */
     public static function meanDeviation(array $x, float $mean = null) : float
     {
-        $mean = isset($mean) ? $mean : Average::arithmeticMean($x);
+        $mean = $mean !== null ? $mean : Average::arithmeticMean($x);
         $sum  = 0.0;
 
         foreach ($x as $xi) {
@@ -256,7 +256,7 @@ class MeasureOfDispersion
      */
     public static function meanAbsoluteDeviation(array $x, float $mean = null) : float
     {
-        $mean = isset($mean) ? $mean : Average::arithmeticMean($x);
+        $mean = $mean !== null ? $mean : Average::arithmeticMean($x);
         $sum  = 0.0;
 
         foreach ($x as $xi) {
@@ -278,7 +278,7 @@ class MeasureOfDispersion
      */
     public static function squaredMeanDeviation(array $x, float $mean = null) : float
     {
-        $mean = isset($mean) ? $mean : Average::arithmeticMean($x);
+        $mean = $mean !== null ? $mean : Average::arithmeticMean($x);
         $sum  = 0.0;
 
         foreach ($x as $xi) {
diff --git a/Message/Http/Request.php b/Message/Http/Request.php
index bf1fe9973..b92895263 100644
--- a/Message/Http/Request.php
+++ b/Message/Http/Request.php
@@ -97,7 +97,7 @@ class Request extends RequestAbstract
      */
     private function init() /* : void */
     {
-        if (!isset($this->uri)) {
+        if ($this->uri === null) {
             $this->initCurrentRequest();
             $this->lock();
             $this->cleanupGlobals();
@@ -263,7 +263,7 @@ class Request extends RequestAbstract
      */
     public function getRequestInfo() : array
     {
-        if (!isset($this->info)) {
+        if ($this->info === null) {
             $this->info['browser'] = $this->getBrowser();
             $this->info['os']      = $this->getOS();
         }
@@ -280,7 +280,7 @@ class Request extends RequestAbstract
      */
     public function getBrowser() : string
     {
-        if (!isset($this->browser)) {
+        if ($this->browser === null) {
             $arr           = BrowserType::getConstants();
             $httpUserAgent = strtolower($_SERVER['HTTP_USER_AGENT']);
 
@@ -321,7 +321,7 @@ class Request extends RequestAbstract
      */
     public function getOS() : string
     {
-        if (!isset($this->os)) {
+        if ($this->os === null) {
             $arr           = OSType::getConstants();
             $httpUserAgent = strtolower($_SERVER['HTTP_USER_AGENT']);
 
@@ -444,7 +444,7 @@ class Request extends RequestAbstract
      */
     public function getMethod() : string
     {
-        if (!isset($this->method)) {
+        if ($this->method === null) {
             $this->method = $_SERVER['REQUEST_METHOD'] ?? RequestMethod::GET;
         }
 
diff --git a/Message/Mail/EmailAbstract.php b/Message/Mail/EmailAbstract.php
index 0a6b58ac1..91edac091 100644
--- a/Message/Mail/EmailAbstract.php
+++ b/Message/Mail/EmailAbstract.php
@@ -127,7 +127,7 @@ class EmailAbstract
      */
     public function disconnect() /* : void */
     {
-        if (!isset($this->con)) {
+        if ($this->con === null) {
             imap_close($this->con);
             $this->con = null;
         }
@@ -148,7 +148,7 @@ class EmailAbstract
         $this->mailbox = substr($this->mailbox, 0, -1) . ($this->ssl ? '/ssl/validate-cert' : '/novalidate-cert') . '}';
 
         // /novalidate-cert
-        if (!isset($this->con)) {
+        if ($this->con === null) {
             $this->con = imap_open($this->mailbox . 'INBOX', $user, $pass);
         }
     }
diff --git a/Message/RequestAbstract.php b/Message/RequestAbstract.php
index b6a3902e7..a1492d3b8 100644
--- a/Message/RequestAbstract.php
+++ b/Message/RequestAbstract.php
@@ -203,7 +203,7 @@ abstract class RequestAbstract implements MessageInterface
      */
     public function getData($key = null)
     {
-        if (!isset($key)) {
+        if ($key === null) {
             return $this->data;
         }
 
diff --git a/Module/ModuleManager.php b/Module/ModuleManager.php
index c10c687f3..0b225c055 100644
--- a/Module/ModuleManager.php
+++ b/Module/ModuleManager.php
@@ -140,7 +140,7 @@ class ModuleManager
      */
     public function getUriLoad(RequestAbstract $request) : array
     {
-        if (!isset($this->uriLoad)) {
+        if ($this->uriLoad === null) {
             switch ($this->app->dbPool->get('select')->getType()) {
                 case DatabaseType::MYSQL:
                     $uriHash = $request->getHash();
@@ -233,7 +233,7 @@ class ModuleManager
      */
     public function getAllModules() : array
     {
-        if (!isset($this->all)) {
+        if ($this->all === null) {
             chdir($this->modulePath);
             $files = glob('*', GLOB_ONLYDIR);
             $c     = count($files);
diff --git a/Stdlib/Graph/BinaryTree.php b/Stdlib/Graph/BinaryTree.php
index 7284f2165..cc6f2f387 100644
--- a/Stdlib/Graph/BinaryTree.php
+++ b/Stdlib/Graph/BinaryTree.php
@@ -155,11 +155,11 @@ class BinaryTree extends Tree
         $left                         = $this->getLeft($node);
         $right                        = $this->getRight($node);
 
-        if (isset($left)) {
+        if ($left !== null) {
             $this->getVerticalOrder($left, $horizontalDistance - 1, $order);
         }
 
-        if (isset($right)) {
+        if ($right !== null) {
             $this->getVerticalOrder($right, $horizontalDistance + 1, $order);
         }
     }
@@ -198,18 +198,18 @@ class BinaryTree extends Tree
      */
     public function isSymmetric(Node $node1 = null, Node $node2 = null) : bool
     {
-        if (!isset($node1) && !isset($node2)) {
+        if ($node1 === null && $node2 === null) {
             return true;
         }
 
         $left1  = $this->getLeft($node1);
         $right1 = $this->getRight($node1);
 
-        $left2  = isset($node2) ? $this->getLeft($node1) : $this->getLeft($node2);
-        $right2 = isset($node2) ? $this->getRight($node1) : $this->getRight($node2);
+        $left2  = $node2 !== null ? $this->getLeft($node1) : $this->getLeft($node2);
+        $right2 = $node2 !== null ? $this->getRight($node1) : $this->getRight($node2);
 
         // todo: compare values? true symmetry requires the values to be the same
-        if (isset($node1, $node2)) {
+        if ($node1 !== null && $node2 !== null) {
             return $this->isSymmetric($left1, $right2) && $this->isSymmetric($right1, $left2);
         }
 
diff --git a/Stdlib/Graph/Tree.php b/Stdlib/Graph/Tree.php
index b874c0f87..744e5a063 100644
--- a/Stdlib/Graph/Tree.php
+++ b/Stdlib/Graph/Tree.php
@@ -74,7 +74,7 @@ class Tree extends Graph
     {
         $currentNode = $node ?? $this->root;
 
-        if (!isset($currentNode)) {
+        if ($currentNode === null) {
             return 0;
         }
 
@@ -101,7 +101,7 @@ class Tree extends Graph
     {
         $currentNode = $node ?? $this->root;
 
-        if (!isset($currentNode)) {
+        if ($currentNode === null) {
             return 0;
         }
 
diff --git a/System/File/Ftp/Directory.php b/System/File/Ftp/Directory.php
index f138d6139..1b3ad5271 100644
--- a/System/File/Ftp/Directory.php
+++ b/System/File/Ftp/Directory.php
@@ -300,7 +300,7 @@ class Directory extends FileAbstract implements DirectoryInterface
      */
     public function offsetSet($offset, $value)
     {
-        if (!isset($offset)) {
+        if ($offset === null) {
             $this->addNode($value);
         } else {
             $this->nodes[$offset] = $value;
diff --git a/System/File/Ftp/FtpStorage.php b/System/File/Ftp/FtpStorage.php
index 9c01a9137..b39fe227c 100644
--- a/System/File/Ftp/FtpStorage.php
+++ b/System/File/Ftp/FtpStorage.php
@@ -39,7 +39,7 @@ class FtpStorage extends StorageAbstract
 
     public static function getInstance() : StorageAbstract
     {
-        if (!isset(self::$instance)) {
+        if (self::$instance === null) {
             self::$instance = new self();
         }
 
diff --git a/System/File/Local/Directory.php b/System/File/Local/Directory.php
index 49911fe02..ab41e5ece 100644
--- a/System/File/Local/Directory.php
+++ b/System/File/Local/Directory.php
@@ -470,7 +470,7 @@ class Directory extends FileAbstract implements DirectoryInterface
      */
     public function offsetSet($offset, $value)
     {
-        if (!isset($offset)) {
+        if ($offset === null) {
             $this->addNode($value);
         } else {
             $this->nodes[$offset] = $value;
diff --git a/System/File/Local/LocalStorage.php b/System/File/Local/LocalStorage.php
index e511112b8..d7e1df229 100644
--- a/System/File/Local/LocalStorage.php
+++ b/System/File/Local/LocalStorage.php
@@ -55,7 +55,7 @@ class LocalStorage extends StorageAbstract
      */
     public static function getInstance() : StorageAbstract
     {
-        if (!isset(self::$instance)) {
+        if (self::$instance === null) {
             self::$instance = new self();
         }
 
diff --git a/Uri/Http.php b/Uri/Http.php
index 586c7d75b..ae5535746 100644
--- a/Uri/Http.php
+++ b/Uri/Http.php
@@ -275,7 +275,7 @@ class Http implements UriInterface
      */
     public function getQuery(string $key = null)  : string
     {
-        if (isset($key)) {
+        if ($key !== null) {
             $key = strtolower($key);
 
             return $this->query[$key] ?? '';
@@ -338,7 +338,7 @@ class Http implements UriInterface
     public function getAuthority() : string
     {
         return ($this->getUser() !== '' ? $this->getUser() . '@' : '') . $this->host 
-            . (isset($this->port) && $this->port !== 0 ? ':' . $this->port : '');
+            . ($this->port !== null && $this->port !== 0 ? ':' . $this->port : '');
     }
 
     /**
diff --git a/Utils/Git/Repository.php b/Utils/Git/Repository.php
index 9cc8ca9d1..6c951112d 100644
--- a/Utils/Git/Repository.php
+++ b/Utils/Git/Repository.php
@@ -246,7 +246,7 @@ class Repository
             throw new \Exception('Already repository');
         }
 
-        if (isset($source)) {
+        if ($source !== null) {
             return stripos($source, '//') !== false ? $this->cloneRemote($source) : $this->cloneFrom($source);
         }
 
@@ -688,11 +688,11 @@ class Repository
      */
     public function getContributors(\DateTime $start = null, \DateTime $end = null) : array
     {
-        if (!isset($start)) {
+        if ($start === null) {
             $start = new \DateTime('1970-12-31');
         }
 
-        if (!isset($end)) {
+        if ($end === null) {
             $end = new \DateTime('now');
         }
 
@@ -727,11 +727,11 @@ class Repository
      */
     public function getCommitsCount(\DateTime $start = null, \DateTime $end = null) : array
     {
-        if (!isset($start)) {
+        if ($start === null) {
             $start = new \DateTime('1970-12-31');
         }
 
-        if (!isset($end)) {
+        if ($end === null) {
             $end = new \DateTime('now');
         }
 
@@ -760,11 +760,11 @@ class Repository
      */
     public function getAdditionsRemovalsByContributor(Author $author, \DateTime $start = null, \DateTime $end = null) : array
     {
-        if (!isset($start)) {
+        if ($start === null) {
             $start = new \DateTime('1900-01-01');
         }
 
-        if (!isset($end)) {
+        if ($end === null) {
             $end = new \DateTime('now');
         }
 
@@ -811,15 +811,15 @@ class Repository
      */
     public function getCommitsBy(\DateTime $start = null, \DateTime $end = null, Author $author = null) : array
     {
-        if (!isset($start)) {
+        if ($start === null) {
             $start = new \DateTime('1970-12-31');
         }
 
-        if (!isset($end)) {
+        if ($end === null) {
             $end = new \DateTime('now');
         }
 
-        if (!isset($author)) {
+        if ($author === null) {
             $author = '';
         } else {
             $author = ' --author=' . escapeshellarg($author->getName()) . '';
diff --git a/Utils/Parser/Markdown/Markdown.php b/Utils/Parser/Markdown/Markdown.php
index 2bf675ca5..a0833ba79 100644
--- a/Utils/Parser/Markdown/Markdown.php
+++ b/Utils/Parser/Markdown/Markdown.php
@@ -170,7 +170,7 @@ class Markdown
             if (isset($currentBlock['continuable'])) {
                 $block = self::{'block' . $currentBlock['type'] . 'Continue'}($lineArray, $currentBlock);
 
-                if (isset($block)) {
+                if ($block !== null) {
                     $currentBlock = $block;
 
                     continue;
@@ -191,7 +191,7 @@ class Markdown
             foreach ($blockTypes as $blockType) {
                 $block = self::{'block' . $blockType}($lineArray, $currentBlock);
 
-                if (isset($block)) {
+                if ($block !== null) {
                     $block['type'] = $blockType;
 
                     if (!isset($block['identified'])) {
@@ -243,7 +243,7 @@ class Markdown
 
     protected static function blockCode(array $lineArray, array $block = null) /* : ?array */
     {
-        if (isset($block) && !isset($block['type']) && !isset($block['interrupted'])) {
+        if ($block !== null && !isset($block['type']) && !isset($block['interrupted'])) {
             return;
         }
 
@@ -676,7 +676,7 @@ class Markdown
             foreach (self::$inlineTypes[$marker] as $inlineType) {
                 $inline = self::{'inline' . $inlineType}($excerptArray);
 
-                if (!isset($inline)) {
+                if ($inline === null) {
                     continue;
                 }
 
@@ -794,7 +794,7 @@ class Markdown
         $excerpt['text'] = substr($excerpt['text'], 1);
         $link            = self::inlineLink($excerpt);
 
-        if (!isset($link)) {
+        if ($link === null) {
             return;
         }
 
diff --git a/Utils/Parser/Php/ArrayParser.php b/Utils/Parser/Php/ArrayParser.php
index 192b66bf4..af90f6490 100644
--- a/Utils/Parser/Php/ArrayParser.php
+++ b/Utils/Parser/Php/ArrayParser.php
@@ -68,7 +68,7 @@ class ArrayParser
             return '"' . $value . '"';
         } elseif (is_scalar($value)) {
             return (string) $value;
-        } elseif (!isset($value)) {
+        } elseif ($value === null) {
             return 'null';
         } elseif (is_bool($value)) {
             return $value ? 'true' : 'false';
diff --git a/Utils/TaskSchedule/Interval.php b/Utils/TaskSchedule/Interval.php
index 7ac01ceac..bde6b0083 100644
--- a/Utils/TaskSchedule/Interval.php
+++ b/Utils/TaskSchedule/Interval.php
@@ -100,7 +100,7 @@ class Interval implements \Serializable
     {
         $this->start = new \DateTime('now');
 
-        if (isset($interval)) {
+        if ($interval !== null) {
             $this->unserialize($interval);
         }
     }
diff --git a/Validation/Validator.php b/Validation/Validator.php
index 18dcde896..6fb8e5c55 100644
--- a/Validation/Validator.php
+++ b/Validation/Validator.php
@@ -39,7 +39,7 @@ final class Validator extends ValidatorAbstract
      */
     public static function isValid($var, array $constraints = null) : bool
     {
-        if (!isset($constraints)) {
+        if ($constraints === null) {
             return true;
         }
 
diff --git a/Views/View.php b/Views/View.php
index d7702771c..8b12a16c4 100644
--- a/Views/View.php
+++ b/Views/View.php
@@ -85,7 +85,7 @@ class View extends ViewAbstract
         $this->app      = $app;
         $this->request  = $request;
         $this->response = $response;
-        $this->l11n     = isset($response) ? $response->getHeader()->getL11n() : null;
+        $this->l11n     = $response !== null ? $response->getHeader()->getL11n() : null;
     }
 
     /**
@@ -174,7 +174,7 @@ class View extends ViewAbstract
      */
     public function getText($translation, string $module = null, string $theme = null) : string
     {
-        if (!isset($module)) {
+        if ($module === null) {
             $match = '/Modules/';
 
             if (($start = strripos($this->template, $match)) === false) {
@@ -186,7 +186,7 @@ class View extends ViewAbstract
             $module = substr($this->template, $start, $end - $start);
         }
 
-        if (!isset($theme)) {
+        if ($theme === null) {
             $match = '/Theme/';
 
             if (($start = strripos($this->template, $match)) === false) {