diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 4cc6424..7bd9715 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -797,7 +797,7 @@ function template_preprocess_image(&$variables) {
     if (isset($variables[$key])) {
       // If the property has already been defined in the attributes,
       // do not override, including NULL.
-      if (array_key_exists($key, $variables['attributes'])) {
+      if (isset($variables['attributes'][$key]) || array_key_exists($key, $variables['attributes'])) {
         continue;
       }
       $variables['attributes'][$key] = $variables[$key];
diff --git a/core/lib/Drupal/Component/Assertion/Inspector.php b/core/lib/Drupal/Component/Assertion/Inspector.php
index 9b04e33..78a9c3d 100644
--- a/core/lib/Drupal/Component/Assertion/Inspector.php
+++ b/core/lib/Drupal/Component/Assertion/Inspector.php
@@ -212,7 +212,7 @@ public static function assertAllHaveKey() {
     if (static::assertTraversable($traversable)) {
       foreach ($traversable as $member) {
         foreach ($args as $key) {
-          if (!array_key_exists($key, $member)) {
+          if (!(isset($member[$key]) || array_key_exists($key, $member))) {
             return FALSE;
           }
         }
diff --git a/core/lib/Drupal/Component/Datetime/DateTimePlus.php b/core/lib/Drupal/Component/Datetime/DateTimePlus.php
index b636262..51f36bb 100644
--- a/core/lib/Drupal/Component/Datetime/DateTimePlus.php
+++ b/core/lib/Drupal/Component/Datetime/DateTimePlus.php
@@ -575,7 +575,7 @@ public static function checkArray($array) {
     $valid_time = TRUE;
     // Check for a valid date using checkdate(). Only values that
     // meet that test are valid.
-    if (array_key_exists('year', $array) && array_key_exists('month', $array) && array_key_exists('day', $array)) {
+    if (isset($array['year']) && isset($array['month']) && isset($array['day'])) {
       if (@checkdate($array['month'], $array['day'], $array['year'])) {
         $valid_date = TRUE;
       }
@@ -583,7 +583,7 @@ public static function checkArray($array) {
     // Testing for valid time is reversed. Missing time is OK,
     // but incorrect values are not.
     foreach (['hour', 'minute', 'second'] as $key) {
-      if (array_key_exists($key, $array)) {
+      if (isset($array[$key]) || array_key_exists($key, $array)) {
         $value = $array[$key];
         switch ($key) {
           case 'hour':
diff --git a/core/lib/Drupal/Component/Utility/DiffArray.php b/core/lib/Drupal/Component/Utility/DiffArray.php
index 825648e..1eea778 100644
--- a/core/lib/Drupal/Component/Utility/DiffArray.php
+++ b/core/lib/Drupal/Component/Utility/DiffArray.php
@@ -29,7 +29,7 @@ public static function diffAssocRecursive(array $array1, array $array2) {
 
     foreach ($array1 as $key => $value) {
       if (is_array($value)) {
-        if (!array_key_exists($key, $array2) || !is_array($array2[$key])) {
+        if (!is_array($array2[$key]) || !(isset($array2[$key]) || array_key_exists($key, $array2))) {
           $difference[$key] = $value;
         }
         else {
@@ -39,7 +39,7 @@ public static function diffAssocRecursive(array $array1, array $array2) {
           }
         }
       }
-      elseif (!array_key_exists($key, $array2) || $array2[$key] !== $value) {
+      elseif (!(isset($array2[$key]) || array_key_exists($key, $array2)) || $array2[$key] !== $value) {
         $difference[$key] = $value;
       }
     }
diff --git a/core/lib/Drupal/Component/Utility/NestedArray.php b/core/lib/Drupal/Component/Utility/NestedArray.php
index c11d6a6..421a624 100644
--- a/core/lib/Drupal/Component/Utility/NestedArray.php
+++ b/core/lib/Drupal/Component/Utility/NestedArray.php
@@ -69,7 +69,7 @@ class NestedArray {
   public static function &getValue(array &$array, array $parents, &$key_exists = NULL) {
     $ref = &$array;
     foreach ($parents as $parent) {
-      if (is_array($ref) && array_key_exists($parent, $ref)) {
+      if (is_array($ref) && (isset($ref[$parent]) || array_key_exists($parent, $ref))) {
         $ref = &$ref[$parent];
       }
       else {
@@ -219,7 +219,7 @@ public static function setValue(array &$array, array $parents, $value, $force =
   public static function unsetValue(array &$array, array $parents, &$key_existed = NULL) {
     $unset_key = array_pop($parents);
     $ref = &self::getValue($array, $parents, $key_existed);
-    if ($key_existed && is_array($ref) && array_key_exists($unset_key, $ref)) {
+    if ($key_existed && is_array($ref) && (isset($ref[$unset_key]) || array_key_exists($unset_key, $ref))) {
       $key_existed = TRUE;
       unset($ref[$unset_key]);
     }
diff --git a/core/lib/Drupal/Core/Config/InstallStorage.php b/core/lib/Drupal/Core/Config/InstallStorage.php
index 9fea3df..8e1805a 100644
--- a/core/lib/Drupal/Core/Config/InstallStorage.php
+++ b/core/lib/Drupal/Core/Config/InstallStorage.php
@@ -92,7 +92,8 @@ public function getFilePath($name) {
    * {@inheritdoc}
    */
   public function exists($name) {
-    return array_key_exists($name, $this->getAllFolders());
+    $all_folders = $this->getAllFolders();
+    return isset($all_folders[$name]) || array_key_exists($name, $all_folders);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php b/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
index 1911d6a..41eef98 100644
--- a/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
@@ -181,7 +181,7 @@ protected function createFieldSql($name, $spec) {
     }
 
     // $spec['default'] can be NULL, so we explicitly check for the key here.
-    if (array_key_exists('default', $spec)) {
+    if (isset($spec['default']) || array_key_exists('default', $spec)) {
       $sql .= ' DEFAULT ' . $this->escapeDefaultValue($spec['default']);
     }
 
diff --git a/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php b/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php
index 4586d01..43e4701 100644
--- a/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php
+++ b/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php
@@ -328,7 +328,9 @@ protected function createFieldSql($name, $spec) {
         $sql .= ' NULL';
       }
     }
-    if (array_key_exists('default', $spec)) {
+
+    // $spec['default'] can be NULL, so we explicitly check for the key here.
+    if (isset($spec['default']) || array_key_exists('default', $spec)) {
       $default = $this->escapeDefaultValue($spec['default']);
       $sql .= " default $default";
     }
diff --git a/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php b/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php
index d999450..c1960a8 100644
--- a/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php
+++ b/core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php
@@ -184,7 +184,7 @@ private function parseDefinition($id, $service, $file)
             $definition->setAbstract($service['abstract']);
         }
 
-        if (array_key_exists('deprecated', $service)) {
+        if (isset($service['deprecated']) || array_key_exists('deprecated', $service)) {
             $definition->setDeprecated(true, $service['deprecated']);
         }
 
diff --git a/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php b/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php
index 9d6b8d6..ffb5c5f 100644
--- a/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php
@@ -108,7 +108,7 @@ protected function initFieldValues(ContentEntityInterface $entity, array $values
         if (isset($values[$name])) {
           $entity->$name = $values[$name];
         }
-        elseif (!array_key_exists($name, $values)) {
+        elseif (!(isset($values[$name]) || array_key_exists($name, $values))) {
           $entity->get($name)->applyDefaultValue();
         }
       }
diff --git a/core/lib/Drupal/Core/Entity/KeyValueStore/KeyValueEntityStorage.php b/core/lib/Drupal/Core/Entity/KeyValueStore/KeyValueEntityStorage.php
index de77e09..7802bee 100644
--- a/core/lib/Drupal/Core/Entity/KeyValueStore/KeyValueEntityStorage.php
+++ b/core/lib/Drupal/Core/Entity/KeyValueStore/KeyValueEntityStorage.php
@@ -100,7 +100,7 @@ public function doCreate(array $values = []) {
         if (isset($values[$name])) {
           $entity->$name = $values[$name];
         }
-        elseif (!array_key_exists($name, $values)) {
+        elseif (!(isset($values[$name]) || array_key_exists($name, $values))) {
           $entity->get($name)->applyDefaultValue();
         }
         unset($values[$name]);
diff --git a/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php
index 1172406..5a34c6d 100644
--- a/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php
+++ b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php
@@ -651,7 +651,7 @@ protected function buildPropertyQuery(QueryInterface $entity_query, array $value
       //   https://www.drupal.org/node/1866330.
       // Default to the original entity language if not explicitly specified
       // otherwise.
-      if (!array_key_exists($this->defaultLangcodeKey, $values)) {
+      if (!(isset($values[$this->defaultLangcodeKey]) || array_key_exists($this->defaultLangcodeKey, $values))) {
         $values[$this->defaultLangcodeKey] = 1;
       }
       // If the 'default_langcode' flag is explicitly not set, we do not care
diff --git a/core/lib/Drupal/Core/Field/FieldConfigBase.php b/core/lib/Drupal/Core/Field/FieldConfigBase.php
index e53e84b..078d9d6 100644
--- a/core/lib/Drupal/Core/Field/FieldConfigBase.php
+++ b/core/lib/Drupal/Core/Field/FieldConfigBase.php
@@ -356,7 +356,7 @@ public function setSettings(array $settings) {
    * {@inheritdoc}
    */
   public function getSetting($setting_name) {
-    if (array_key_exists($setting_name, $this->settings)) {
+    if (isset($this->settings[$setting_name]) || array_key_exists($setting_name, $this->settings)) {
       return $this->settings[$setting_name];
     }
     else {
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
index f9922b9..cf2d4bd 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
@@ -175,13 +175,13 @@ public function setValue($values, $notify = TRUE) {
       // Support setting the field item with only one property, but make sure
       // values stay in sync if only property is passed.
       // NULL is a valid value, so we use array_key_exists().
-      if (is_array($values) && array_key_exists('target_id', $values) && !isset($values['entity'])) {
+      if (is_array($values) && (isset($values['target_id']) || array_key_exists('target_id', $values)) && !isset($values['entity'])) {
         $this->onChange('target_id', FALSE);
       }
-      elseif (is_array($values) && !array_key_exists('target_id', $values) && isset($values['entity'])) {
+      elseif (is_array($values) && !(isset($values['target_id']) || array_key_exists('target_id', $values)) && isset($values['entity'])) {
         $this->onChange('entity', FALSE);
       }
-      elseif (is_array($values) && array_key_exists('target_id', $values) && isset($values['entity'])) {
+      elseif (is_array($values) && (isset($values['target_id']) || array_key_exists('target_id', $values)) && isset($values['entity'])) {
         // If both properties are passed, verify the passed values match. The
         // only exception we allow is when we have a new entity: in this case
         // its actual id and target_id will be different, due to the new entity
@@ -328,10 +328,10 @@ public function fieldSettingsForm(array $form, FormStateInterface $form_state) {
       // We only display base plugins (e.g. 'default', 'views', ...) and not
       // entity type specific plugins (e.g. 'default:node', 'default:user',
       // ...).
-      if (array_key_exists($selection_group_id, $selection_plugins[$selection_group_id])) {
+      if (isset($selection_plugins[$selection_group_id][$selection_group_id]) || array_key_exists($selection_group_id, $selection_plugins[$selection_group_id])) {
         $handlers_options[$selection_group_id] = Html::escape($selection_plugins[$selection_group_id][$selection_group_id]['label']);
       }
-      elseif (array_key_exists($selection_group_id . ':' . $this->getSetting('target_type'), $selection_plugins[$selection_group_id])) {
+      elseif (isset($selection_plugins[$selection_group_id][$selection_group_id . ':' . $this->getSetting('target_type')]) || array_key_exists($selection_group_id . ':' . $this->getSetting('target_type'), $selection_plugins[$selection_group_id])) {
         $selection_group_plugin = $selection_group_id . ':' . $this->getSetting('target_type');
         $handlers_options[$selection_group_plugin] = Html::escape($selection_plugins[$selection_group_id][$selection_group_plugin]['base_plugin_label']);
       }
diff --git a/core/lib/Drupal/Core/Field/PluginSettingsBase.php b/core/lib/Drupal/Core/Field/PluginSettingsBase.php
index 073bd2c..d0bd132 100644
--- a/core/lib/Drupal/Core/Field/PluginSettingsBase.php
+++ b/core/lib/Drupal/Core/Field/PluginSettingsBase.php
@@ -58,7 +58,7 @@ public function getSettings() {
    */
   public function getSetting($key) {
     // Merge defaults if we have no value for the key.
-    if (!$this->defaultSettingsMerged && !array_key_exists($key, $this->settings)) {
+    if (!$this->defaultSettingsMerged && !(isset($this->settings[$key]) || array_key_exists($key, $this->settings))) {
       $this->mergeDefaults();
     }
     return isset($this->settings[$key]) ? $this->settings[$key] : NULL;
diff --git a/core/lib/Drupal/Core/ImageToolkit/ImageToolkitOperationBase.php b/core/lib/Drupal/Core/ImageToolkit/ImageToolkitOperationBase.php
index c9a93ff..8c49812 100644
--- a/core/lib/Drupal/Core/ImageToolkit/ImageToolkitOperationBase.php
+++ b/core/lib/Drupal/Core/ImageToolkit/ImageToolkitOperationBase.php
@@ -106,7 +106,7 @@ protected function prepareArguments(array $arguments) {
       $argument += ['required' => TRUE];
       // Check if the argument is required and, if so, has been provided.
       if ($argument['required']) {
-        if (!array_key_exists($id, $arguments)) {
+        if (!(isset($arguments[$id]) || !array_key_exists($id, $arguments))) {
           // If the argument is required throw an exception.
           throw new \InvalidArgumentException("Argument '$id' expected by plugin '{$this->getPluginId()}' but not passed");
         }
@@ -115,14 +115,14 @@ protected function prepareArguments(array $arguments) {
         // Optional arguments require a 'default' value.
         // We check this even if the argument is provided by the caller, as we
         // want to fail fast here, i.e. at development time.
-        if (!array_key_exists('default', $argument)) {
+        if (!(isset($argument['default']) || !array_key_exists('default', $argument))) {
           // The plugin did not define a default, so throw a plugin exception,
           // not an invalid argument exception.
           throw new InvalidPluginDefinitionException("Default for argument '$id' expected by plugin '{$this->getPluginId()}' but not defined");
         }
 
         // Use the default value if the argument is not passed in.
-        if (!array_key_exists($id, $arguments)) {
+        if (!(isset($arguments[$id]) || !array_key_exists($id, $arguments))) {
           $arguments[$id] = $argument['default'];
         }
       }
diff --git a/core/lib/Drupal/Core/KeyValueStore/MemoryStorage.php b/core/lib/Drupal/Core/KeyValueStore/MemoryStorage.php
index d6a6ca9..766665c 100644
--- a/core/lib/Drupal/Core/KeyValueStore/MemoryStorage.php
+++ b/core/lib/Drupal/Core/KeyValueStore/MemoryStorage.php
@@ -18,14 +18,14 @@ class MemoryStorage extends StorageBase {
    * {@inheritdoc}
    */
   public function has($key) {
-    return array_key_exists($key, $this->data);
+    return isset($this->data[$key]) || array_key_exists($key, $this->data);
   }
 
   /**
    * {@inheritdoc}
    */
   public function get($key, $default = NULL) {
-    return array_key_exists($key, $this->data) ? $this->data[$key] : $default;
+    return (isset($this->data[$key]) || array_key_exists($key, $this->data)) ? $this->data[$key] : $default;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Path/AliasWhitelist.php b/core/lib/Drupal/Core/Path/AliasWhitelist.php
index 9286c95..2e0c144 100644
--- a/core/lib/Drupal/Core/Path/AliasWhitelist.php
+++ b/core/lib/Drupal/Core/Path/AliasWhitelist.php
@@ -92,7 +92,7 @@ public function get($offset) {
         return TRUE;
       }
     }
-    elseif (array_key_exists($offset, $this->storage)) {
+    elseif (isset($this->storage[$offset]) || array_key_exists($offset, $this->storage)) {
       return $this->resolveCacheMiss($offset);
     }
   }
diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index 243a9af..ad6ffc9 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -339,7 +339,7 @@ public static function preRenderAjaxForm($element) {
         'options' => ['query' => []],
         'dialogType' => 'ajax',
       ];
-      if (array_key_exists('callback', $settings) && !isset($settings['url'])) {
+      if ((isset($settings['callback']) || array_key_exists('callback', $settings)) && !isset($settings['url'])) {
         $settings['url'] = Url::fromRoute('<current>');
         // Add all the current query parameters in order to ensure that we build
         // the same form on the AJAX POST requests. For example,
diff --git a/core/lib/Drupal/Core/Routing/UrlGenerator.php b/core/lib/Drupal/Core/Routing/UrlGenerator.php
index 63dcd47..6b2ca9e 100644
--- a/core/lib/Drupal/Core/Routing/UrlGenerator.php
+++ b/core/lib/Drupal/Core/Routing/UrlGenerator.php
@@ -195,7 +195,10 @@ protected function doGenerate(array $variables, array $defaults, array $tokens,
     // [ [ 0 => 'text', 1 => '/admin/config' ] ]
     foreach ($tokens as $token) {
       if ('variable' === $token[0]) {
-        if (!$optional || !array_key_exists($token[3], $defaults) || (isset($mergedParams[$token[3]]) && (string) $mergedParams[$token[3]] !== (string) $defaults[$token[3]])) {
+        if (!$optional
+          || !(isset($defaults[$token[3]]) || array_key_exists($token[3], $defaults))
+          || (isset($mergedParams[$token[3]])
+          && (string) $mergedParams[$token[3]] !== (string) $defaults[$token[3]])) {
           // check requirement
           if (!preg_match('#^' . $token[2] . '$#', $mergedParams[$token[3]])) {
             $message = sprintf('Parameter "%s" for route "%s" must match "%s" ("%s" given) to generate a corresponding URL.', $token[3], $name, $token[2], $mergedParams[$token[3]]);
diff --git a/core/lib/Drupal/Core/TypedData/DataDefinition.php b/core/lib/Drupal/Core/TypedData/DataDefinition.php
index 52a4394..a8e1cf5 100644
--- a/core/lib/Drupal/Core/TypedData/DataDefinition.php
+++ b/core/lib/Drupal/Core/TypedData/DataDefinition.php
@@ -295,9 +295,7 @@ public function addConstraint($constraint_name, $options = NULL) {
    * @todo: Remove in https://www.drupal.org/node/1928868.
    */
   public function offsetExists($offset) {
-    // PHP's array access does not work correctly with isset(), so we have to
-    // bake isset() in here. See https://bugs.php.net/bug.php?id=41727.
-    return array_key_exists($offset, $this->definition) && isset($this->definition[$offset]);
+    return isset($this->definition[$offset]);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php b/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php
index 4f756da..9f28fe0 100644
--- a/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php
+++ b/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php
@@ -130,7 +130,7 @@ public function set($index, $value) {
    * {@inheritdoc}
    */
   public function removeItem($index) {
-    if (isset($this->list) && array_key_exists($index, $this->list)) {
+    if (isset($this->list) && (isset($this->list[$index]) || array_key_exists($index, $this->list))) {
       // Remove the item, and reassign deltas.
       unset($this->list[$index]);
       $this->rekey($index);
diff --git a/core/lib/Drupal/Core/Utility/ThemeRegistry.php b/core/lib/Drupal/Core/Utility/ThemeRegistry.php
index bc2ce5a..fe80bdf 100644
--- a/core/lib/Drupal/Core/Utility/ThemeRegistry.php
+++ b/core/lib/Drupal/Core/Utility/ThemeRegistry.php
@@ -96,7 +96,7 @@ public function has($key) {
     // are not registered, just check the existence of the key in the registry.
     // Use array_key_exists() here since a NULL value indicates that the theme
     // hook exists but has not yet been requested.
-    return array_key_exists($key, $this->storage);
+    return isset($this->storage[$key]) || array_key_exists($key, $this->storage);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/ComplexDataConstraint.php b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/ComplexDataConstraint.php
index a020cdc..3b5e575 100644
--- a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/ComplexDataConstraint.php
+++ b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/ComplexDataConstraint.php
@@ -28,7 +28,7 @@ class ComplexDataConstraint extends Constraint {
    */
   public function __construct($options = NULL) {
     // Allow skipping the 'properties' key in the options.
-    if (is_array($options) && !array_key_exists('properties', $options)) {
+    if (is_array($options) && !(isset($options['properties']) || array_key_exists('properties', $options))) {
       $options = ['properties' => $options];
     }
     parent::__construct($options);
diff --git a/core/modules/aggregator/src/Form/SettingsForm.php b/core/modules/aggregator/src/Form/SettingsForm.php
index 48b2dfe..9d4aec5 100644
--- a/core/modules/aggregator/src/Form/SettingsForm.php
+++ b/core/modules/aggregator/src/Form/SettingsForm.php
@@ -156,7 +156,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
     // Call buildConfigurationForm() on the active fetcher and parser.
     foreach (['fetcher', 'parser'] as $type) {
       $active = $config->get($type);
-      if (array_key_exists($active, $this->definitions[$type])) {
+      if (isset($this->definitions[$type][$active]) || array_key_exists($active, $this->definitions[$type])) {
         $instance = $this->managers[$type]->createInstance($active);
         if ($instance instanceof PluginFormInterface) {
           $form = $instance->buildConfigurationForm($form, $form_state);
diff --git a/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
index b2d180a..4adb980 100644
--- a/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -323,7 +323,7 @@ protected function validateVisibility(array $form, FormStateInterface $form_stat
       // All condition plugins use 'negate' as a Boolean in their schema.
       // However, certain form elements may return it as 0/1. Cast here to
       // ensure the data is in the expected type.
-      if (array_key_exists('negate', $values)) {
+      if (isset($values['negate']) || array_key_exists('negate', $values)) {
         $form_state->setValue(['visibility', $condition_id, 'negate'], (bool) $values['negate']);
       }
 
diff --git a/core/modules/content_translation/src/ContentTranslationHandler.php b/core/modules/content_translation/src/ContentTranslationHandler.php
index 7e6a7ca..c343b1d 100644
--- a/core/modules/content_translation/src/ContentTranslationHandler.php
+++ b/core/modules/content_translation/src/ContentTranslationHandler.php
@@ -225,7 +225,8 @@ protected function hasCreatedTime() {
    *   TRUE if translatable field storage definition exists, FALSE otherwise.
    */
   protected function checkFieldStorageDefinitionTranslatability($field_name) {
-    return array_key_exists($field_name, $this->fieldStorageDefinitions) && $this->fieldStorageDefinitions[$field_name]->isTranslatable();
+    return (isset($this->fieldStorageDefinitions[$field_name]) || array_key_exists($field_name, $this->fieldStorageDefinitions))
+      && $this->fieldStorageDefinitions[$field_name]->isTranslatable();
   }
 
   /**
diff --git a/core/modules/content_translation/src/Controller/ContentTranslationController.php b/core/modules/content_translation/src/Controller/ContentTranslationController.php
index c18eab3..0438b7b 100644
--- a/core/modules/content_translation/src/Controller/ContentTranslationController.php
+++ b/core/modules/content_translation/src/Controller/ContentTranslationController.php
@@ -160,7 +160,7 @@ public function overview(RouteMatchInterface $route_match, $entity_type_id = NUL
         ];
 
         $links = &$operations['data']['#links'];
-        if (array_key_exists($langcode, $translations)) {
+        if (isset($translations[$langcode]) || array_key_exists($langcode, $translations)) {
           // Existing translation in the translation set: display status.
           $translation = $entity->getTranslation($langcode);
           $metadata = $manager->getTranslationMetadata($translation);
diff --git a/core/modules/field/src/Entity/FieldStorageConfig.php b/core/modules/field/src/Entity/FieldStorageConfig.php
index 78b3d60..e68b60d 100644
--- a/core/modules/field/src/Entity/FieldStorageConfig.php
+++ b/core/modules/field/src/Entity/FieldStorageConfig.php
@@ -552,11 +552,11 @@ public function getSetting($setting_name) {
     // @todo See getSettings() about potentially statically caching this.
     // We assume here that one call to array_key_exists() is more efficient
     // than calling getSettings() when all we need is a single setting.
-    if (array_key_exists($setting_name, $this->settings)) {
+    if (isset($this->settings[$setting_name]) || array_key_exists($setting_name, $this->settings)) {
       return $this->settings[$setting_name];
     }
     $settings = $this->getSettings();
-    if (array_key_exists($setting_name, $settings)) {
+    if (isset($settings[$setting_name]) || array_key_exists($setting_name, $settings)) {
       return $settings[$setting_name];
     }
     else {
diff --git a/core/modules/file/tests/file_test/file_test.module b/core/modules/file/tests/file_test/file_test.module
index 0baa0cb..c4139d0 100644
--- a/core/modules/file/tests/file_test/file_test.module
+++ b/core/modules/file/tests/file_test/file_test.module
@@ -232,7 +232,7 @@ function file_test_file_url_alter(&$uri) {
       // Serve files with one of the CDN extensions from CDN 1, all others from
       // CDN 2.
       $pathinfo = pathinfo($path);
-      if (array_key_exists('extension', $pathinfo) && in_array($pathinfo['extension'], $cdn_extensions)) {
+      if ((isset($pathinfo['extension']) || array_key_exists('extension', $pathinfo)) && in_array($pathinfo['extension'], $cdn_extensions)) {
         $uri = FILE_URL_TEST_CDN_1 . '/' . $path;
       }
       else {
diff --git a/core/modules/filter/src/Entity/FilterFormat.php b/core/modules/filter/src/Entity/FilterFormat.php
index 9dbaa62..56b3b50 100644
--- a/core/modules/filter/src/Entity/FilterFormat.php
+++ b/core/modules/filter/src/Entity/FilterFormat.php
@@ -313,7 +313,7 @@ public function getHtmlRestrictions() {
             foreach ($intersection as $tag => $attributes) {
               // If the current tag is not whitelisted by the new filter, then
               // it's outside of the intersection.
-              if (!array_key_exists($tag, $new_restrictions['allowed'])) {
+              if (!(isset($new_restrictions['allowed'][$tag]) || array_key_exists($tag, $new_restrictions['allowed']))) {
                 // The exception is the asterisk (which applies to all tags): it
                 // does not need to be whitelisted by every filter in order to be
                 // used; not every filter needs attribute restrictions on all tags.
diff --git a/core/modules/image/src/Plugin/Field/FieldWidget/ImageWidget.php b/core/modules/image/src/Plugin/Field/FieldWidget/ImageWidget.php
index 735cfce..6aa4f8f 100644
--- a/core/modules/image/src/Plugin/Field/FieldWidget/ImageWidget.php
+++ b/core/modules/image/src/Plugin/Field/FieldWidget/ImageWidget.php
@@ -263,7 +263,7 @@ public static function validateRequiredFields($element, FormStateInterface $form
       $image_field = NestedArray::getValue($form_state->getUserInput(), $parents);
       // We check for the array key, so that it can be NULL (like if the user
       // submits the form without using the "upload" button).
-      if (!array_key_exists($field, $image_field)) {
+      if (!(isset($image_field[$field]) || !array_key_exists($field, $image_field))) {
         return;
       }
     }
diff --git a/core/modules/language/src/Form/NegotiationBrowserForm.php b/core/modules/language/src/Form/NegotiationBrowserForm.php
index 5535fe0..c82f466 100644
--- a/core/modules/language/src/Form/NegotiationBrowserForm.php
+++ b/core/modules/language/src/Form/NegotiationBrowserForm.php
@@ -158,7 +158,7 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
       $mappings = $form_state->getValue('mappings');
       foreach ($mappings as $key => $data) {
         // Make sure browser_langcode is unique.
-        if (array_key_exists($data['browser_langcode'], $unique_values)) {
+        if (isset($unique_values[$data['browser_langcode']]) || array_key_exists($data['browser_langcode'], $unique_values)) {
           $form_state->setErrorByName('mappings][new_mapping][browser_langcode', $this->t('Browser language codes must be unique.'));
         }
         elseif (preg_match('/[^a-z\-]/', $data['browser_langcode'])) {
@@ -172,7 +172,7 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
     $data = $form_state->getValue('new_mapping');
     if (!empty($data['browser_langcode'])) {
       // Make sure browser_langcode is unique.
-      if (array_key_exists($data['browser_langcode'], $unique_values)) {
+      if (isset($unique_values[$data['browser_langcode']]) || array_key_exists($data['browser_langcode'], $unique_values)) {
         $form_state->setErrorByName('mappings][' . $key . '][browser_langcode', $this->t('Browser language codes must be unique.'));
       }
       elseif (preg_match('/[^a-z\-]/', $data['browser_langcode'])) {
diff --git a/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationContentEntity.php b/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationContentEntity.php
index 9218899..7af4ded 100644
--- a/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationContentEntity.php
+++ b/core/modules/language/src/Plugin/LanguageNegotiation/LanguageNegotiationContentEntity.php
@@ -91,8 +91,8 @@ public static function create(ContainerInterface $container, array $configuratio
    */
   public function getLangcode(Request $request = NULL) {
     $langcode = $request->query->get(static::QUERY_PARAMETER);
-
-    $language_enabled = array_key_exists($langcode, $this->languageManager->getLanguages());
+    $languages = $this->languageManager->getLanguages();
+    $language_enabled = isset($languages[$langcode]) || array_key_exists($langcode, $languages);
     return $language_enabled ? $langcode : NULL;
   }
 
diff --git a/core/modules/locale/src/LocaleProjectStorage.php b/core/modules/locale/src/LocaleProjectStorage.php
index 05faa17..8253ef0 100644
--- a/core/modules/locale/src/LocaleProjectStorage.php
+++ b/core/modules/locale/src/LocaleProjectStorage.php
@@ -60,7 +60,7 @@ public function getMultiple(array $keys) {
         $values[$key] = $this->cache[$key];
       }
       // Load the value if we don't have an explicit NULL value.
-      elseif (!array_key_exists($key, $this->cache)) {
+      elseif (!(isset($this->cache[$key]) || array_key_exists($key, $this->cache))) {
         $load[] = $key;
       }
     }
diff --git a/core/modules/migrate/src/Plugin/migrate/process/StaticMap.php b/core/modules/migrate/src/Plugin/migrate/process/StaticMap.php
index 1d68f35..2d3e0ed 100644
--- a/core/modules/migrate/src/Plugin/migrate/process/StaticMap.php
+++ b/core/modules/migrate/src/Plugin/migrate/process/StaticMap.php
@@ -124,7 +124,7 @@ public function transform($value, MigrateExecutableInterface $migrate_executable
     }
     $new_value = NestedArray::getValue($this->configuration['map'], $new_value, $key_exists);
     if (!$key_exists) {
-      if (array_key_exists('default_value', $this->configuration)) {
+      if (isset($this->configuration['default_value']) || array_key_exists('default_value', $this->configuration)) {
         if (!empty($this->configuration['bypass'])) {
           throw new MigrateException('Setting both default_value and bypass is invalid.');
         }
diff --git a/core/modules/migrate/src/Plugin/migrate/process/SubProcess.php b/core/modules/migrate/src/Plugin/migrate/process/SubProcess.php
index 5d99955..3ef50e1 100644
--- a/core/modules/migrate/src/Plugin/migrate/process/SubProcess.php
+++ b/core/modules/migrate/src/Plugin/migrate/process/SubProcess.php
@@ -128,7 +128,7 @@ public function transform($value, MigrateExecutableInterface $migrate_executable
         $new_row = new Row($new_value, []);
         $migrate_executable->processRow($new_row, $this->configuration['process']);
         $destination = $new_row->getDestination();
-        if (array_key_exists('key', $this->configuration)) {
+	if (isset($this->configuration['key']) || array_key_exists('key', $this->configuration)) {
           $key = $this->transformKey($key, $migrate_executable, $new_row);
         }
         $return[$key] = $destination;
diff --git a/core/modules/node/src/Plugin/views/wizard/Node.php b/core/modules/node/src/Plugin/views/wizard/Node.php
index 34d4bcf..a58e1e8 100644
--- a/core/modules/node/src/Plugin/views/wizard/Node.php
+++ b/core/modules/node/src/Plugin/views/wizard/Node.php
@@ -245,7 +245,7 @@ protected function buildFilters(&$form, FormStateInterface $form_state) {
       // that is created by the Standard install profile in core and also
       // commonly used by contrib modules; thus, it is most likely to be
       // associated with the "main" free-tagging vocabulary on the site.
-      if (array_key_exists('field_tags', $tag_fields)) {
+      if (isset($tag_fields['field_tags']) || array_key_exists('field_tags', $tag_fields)) {
         $tag_field_name = 'field_tags';
       }
       else {
diff --git a/core/modules/rdf/rdf.module b/core/modules/rdf/rdf.module
index 8d43298..c6c8a7b 100644
--- a/core/modules/rdf/rdf.module
+++ b/core/modules/rdf/rdf.module
@@ -114,7 +114,7 @@ function rdf_get_namespaces() {
   foreach (\Drupal::moduleHandler()->getImplementations('rdf_namespaces') as $module) {
     $function = $module . '_rdf_namespaces';
     foreach ($function() as $prefix => $namespace) {
-      if (array_key_exists($prefix, $namespaces) && $namespace !== $namespaces[$prefix]) {
+      if ((isset($namespaces[$prefix]) || array_key_exists($prefix, $namespaces)) && $namespace !== $namespaces[$prefix]) {
         throw new Exception(t('Tried to map @prefix to @namespace, but @prefix is already mapped to @orig_namespace.', ['@prefix' => $prefix, '@namespace' => $namespace, '@orig_namespace' => $namespaces[$prefix]]));
       }
       else {
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 83d7feb..6a202f7 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -58,7 +58,8 @@ function simpletest_js_alter(&$javascript, AttachedAssetsInterface $assets) {
   // Since SimpleTest is a special use case for the table select, stick the
   // SimpleTest JavaScript above the table select.
   $simpletest = drupal_get_path('module', 'simpletest') . '/simpletest.js';
-  if (array_key_exists($simpletest, $javascript) && array_key_exists('core/misc/tableselect.js', $javascript)) {
+  if ((isset($javascript[$simpletest]) || array_key_exists($simpletest, $javascript))
+    && (isset($javascript['core/misc/tableselect.js']) || array_key_exists('core/misc/tableselect.js', $javascript))) {
     $javascript[$simpletest]['weight'] = $javascript['core/misc/tableselect.js']['weight'] - 1;
   }
 }
diff --git a/core/modules/system/src/Plugin/views/field/BulkForm.php b/core/modules/system/src/Plugin/views/field/BulkForm.php
index 65fdc51..a94f622 100644
--- a/core/modules/system/src/Plugin/views/field/BulkForm.php
+++ b/core/modules/system/src/Plugin/views/field/BulkForm.php
@@ -250,7 +250,8 @@ public function viewsForm(&$form, FormStateInterface $form_state) {
 
     // Add the tableselect javascript.
     $form['#attached']['library'][] = 'core/drupal.tableselect';
-    $use_revision = array_key_exists('revision', $this->view->getQuery()->getEntityTableInfo());
+    $table_info = $this->view->getQuery()->getEntityTableInfo();
+    $use_revision = isset($table_info['revision']) || array_key_exists('revision', $table_info);
 
     // Only add the bulk form options and buttons if there are results.
     if (!empty($this->view->result)) {
diff --git a/core/modules/system/tests/modules/common_test/common_test.module b/core/modules/system/tests/modules/common_test/common_test.module
index 040e40a..26f73b2 100644
--- a/core/modules/system/tests/modules/common_test/common_test.module
+++ b/core/modules/system/tests/modules/common_test/common_test.module
@@ -270,7 +270,7 @@ function common_test_page_attachments_alter(array &$page) {
  */
 function common_test_js_settings_alter(&$settings, AttachedAssetsInterface $assets) {
   // Modify an existing setting.
-  if (array_key_exists('pluralDelimiter', $settings)) {
+  if (isset($settings['pluralDelimiter']) || array_key_exists('pluralDelimiter', $settings)) {
     $settings['pluralDelimiter'] = '☃';
   }
 
diff --git a/core/modules/toolbar/toolbar.module b/core/modules/toolbar/toolbar.module
index 16983d6..98e4061 100644
--- a/core/modules/toolbar/toolbar.module
+++ b/core/modules/toolbar/toolbar.module
@@ -102,7 +102,7 @@ function template_preprocess_toolbar(&$variables) {
         'links' => $element[$key]['tray'],
         'attributes' => new Attribute($attributes),
       ];
-      if (array_key_exists('#heading', $element[$key]['tray'])) {
+      if (isset($element[$key]['tray']['#heading']) || array_key_exists('#heading', $element[$key]['tray'])) {
         $variables['trays'][$key]['label'] = $element[$key]['tray']['#heading'];
       }
     }
diff --git a/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php b/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php
index 3744fea..8c29657 100644
--- a/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php
+++ b/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php
@@ -772,7 +772,7 @@ public function getOption($option) {
       return $this->default_display->getOption($option);
     }
 
-    if (array_key_exists($option, $this->options)) {
+    if (isset($this->options[$option]) || array_key_exists($option, $this->options)) {
       return $this->options[$option];
     }
   }
diff --git a/core/modules/views/src/Plugin/views/join/JoinPluginBase.php b/core/modules/views/src/Plugin/views/join/JoinPluginBase.php
index ccf40f6..7e79bb8 100644
--- a/core/modules/views/src/Plugin/views/join/JoinPluginBase.php
+++ b/core/modules/views/src/Plugin/views/join/JoinPluginBase.php
@@ -341,7 +341,7 @@ protected function buildExtra($info, &$arguments, $table, SelectInterface $selec
     // Figure out the table name. Remember, only use aliases provided
     // if at all possible.
     $join_table = '';
-    if (!array_key_exists('table', $info)) {
+    if (!(isset($info['table']) || array_key_exists('table', $info))) { 
       $join_table = $table['alias'] . '.';
     }
     elseif (isset($info['table'])) {
diff --git a/core/modules/views/src/Plugin/views/join/Subquery.php b/core/modules/views/src/Plugin/views/join/Subquery.php
index f407ffc..579a7f9 100644
--- a/core/modules/views/src/Plugin/views/join/Subquery.php
+++ b/core/modules/views/src/Plugin/views/join/Subquery.php
@@ -60,7 +60,7 @@ public function buildJoin($select_query, $table, $view_query) {
           // Figure out the table name. Remember, only use aliases provided
           // if at all possible.
           $join_table = '';
-          if (!array_key_exists('table', $info)) {
+          if (!(isset($info['table']) || array_key_exists('table', $info))) {
             $join_table = $table['alias'] . '.';
           }
           elseif (isset($info['table'])) {
diff --git a/core/modules/views/src/Plugin/views/query/Sql.php b/core/modules/views/src/Plugin/views/query/Sql.php
index ab056f5..cee232a 100644
--- a/core/modules/views/src/Plugin/views/query/Sql.php
+++ b/core/modules/views/src/Plugin/views/query/Sql.php
@@ -310,7 +310,7 @@ public function addRelationship($alias, JoinPluginBase $join, $base, $link_point
     if (empty($link_point)) {
       $link_point = $this->view->storage->get('base_table');
     }
-    elseif (!array_key_exists($link_point, $this->relationships)) {
+    elseif (!(isset($this->relationships[$link_point]) || array_key_exists($link_point, $this->relationships))) {
       return FALSE;
     }
 
@@ -429,7 +429,7 @@ public function queueTable($table, $relationship = NULL, JoinPluginBase $join =
       $relationship = $this->view->storage->get('base_table');
     }
 
-    if (!array_key_exists($relationship, $this->relationships)) {
+    if (!(isset($this->relationships[$relationship]) || array_key_exists($relationship, $this->relationships))) {
       return FALSE;
     }
 
@@ -538,7 +538,7 @@ public function ensureTable($table, $relationship = NULL, JoinPluginBase $join =
       return $this->tables[$relationship][$table]['alias'];
     }
 
-    if (!array_key_exists($relationship, $this->relationships)) {
+    if (!(isset($this->relationships[$relationship]) || array_key_exists($relationship, $this->relationships))) {
       return FALSE;
     }
 
@@ -608,7 +608,7 @@ protected function ensurePath($table, $relationship = NULL, $join = NULL, $trace
       $relationship = $this->view->storage->get('base_table');
     }
 
-    if (!array_key_exists($relationship, $this->relationships)) {
+    if (!(isset($this->relationships[$relationship]) || array_key_exists($relationship, $this->relationships))) {
       return FALSE;
     }
 
diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index 09a7aad..c9bf217 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -384,7 +384,7 @@ function simpletest_script_parse_args() {
   while ($arg = array_shift($_SERVER['argv'])) {
     if (preg_match('/--(\S+)/', $arg, $matches)) {
       // Argument found.
-      if (array_key_exists($matches[1], $args)) {
+      if (isset($args[$matches[1]]) || array_key_exists($matches[1], $args)) {
         // Argument found in list.
         $previous_arg = $matches[1];
         if (is_bool($args[$previous_arg])) {