diff --git a/classes/components/forms/dashboard/PKPSubmissionFilters.php b/classes/components/forms/dashboard/PKPSubmissionFilters.php
index 1d92ddf662b..49519cf62d2 100644
--- a/classes/components/forms/dashboard/PKPSubmissionFilters.php
+++ b/classes/components/forms/dashboard/PKPSubmissionFilters.php
@@ -61,7 +61,7 @@ protected function isManagerOrAdmin(): bool
         );
     }
 
-    protected function addSectionFields(): self
+    protected function addSectionFields(): static
     {
         if (count($this->sections) === 1) {
             return $this;
@@ -83,7 +83,7 @@ protected function addSectionFields(): self
         ]));
     }
 
-    protected function addAssignedTo(): self
+    protected function addAssignedTo(): static
     {
         if (!$this->isManagerOrAdmin()) {
             return $this;
@@ -113,7 +113,7 @@ protected function addAssignedTo(): self
     }
 
 
-    protected function addCategories(): self
+    protected function addCategories(): static
     {
         if (!$this->categories->count()) {
             return $this;
@@ -152,7 +152,7 @@ protected function addCategories(): self
         return $this->addField(new FieldAutosuggestPreset('categoryIds', $props));
     }
 
-    protected function addDaysSinceLastActivity(): self
+    protected function addDaysSinceLastActivity(): static
     {
         $props = [
             'min' => 0,
diff --git a/classes/controlledVocab/ControlledVocabEntry.php b/classes/controlledVocab/ControlledVocabEntry.php
index d5657c0b791..59745399f5f 100644
--- a/classes/controlledVocab/ControlledVocabEntry.php
+++ b/classes/controlledVocab/ControlledVocabEntry.php
@@ -199,7 +199,7 @@ public function scopeWithSetting(
     /**
      * Get entry related data
      */
-    public function getEntryData(string $locale = null): ?array
+    public function getEntryData(?string $locale = null): ?array
     {
         $multilingualProps = array_flip($this->getMultilingualProps());
         $attributes = Arr::mapWithKeys($this->getSettings(), function (string $prop) use ($locale, $multilingualProps): array {
diff --git a/classes/core/DataObject.php b/classes/core/DataObject.php
index c1f04cc9d90..77bfc8ddc7c 100644
--- a/classes/core/DataObject.php
+++ b/classes/core/DataObject.php
@@ -80,7 +80,7 @@ public function getLocalizedData(string $key, ?string $preferredLocale = null, ?
     /**
      * Get the value of a data variable.
      */
-    public function &getData(string $key, string $locale = null)
+    public function &getData(string $key, ?string $locale = null)
     {
         if (is_null($locale)) {
             if (array_key_exists($key, $this->_data)) {
diff --git a/classes/plugins/ThemePlugin.php b/classes/plugins/ThemePlugin.php
index 6bf2630ee68..91e20c39347 100644
--- a/classes/plugins/ThemePlugin.php
+++ b/classes/plugins/ThemePlugin.php
@@ -496,7 +496,7 @@ public function getOption($name)
     /**
      * Get the localized value of an option
      */
-    public function getLocalizedOption(string $name, string $preferredLocale = null, string &$selectedLocale = null): mixed
+    public function getLocalizedOption(string $name, ?string $preferredLocale = null, ?string &$selectedLocale = null): mixed
     {
         return $this->getBestLocalizedData($this->getOption($name), $preferredLocale, $selectedLocale);
     }
diff --git a/classes/submission/DAO.php b/classes/submission/DAO.php
index b9c1980c88f..32a7d63ba73 100644
--- a/classes/submission/DAO.php
+++ b/classes/submission/DAO.php
@@ -162,7 +162,7 @@ public function getIdsBySetting(string $settingName, $settingValue, int $context
      * (see <http://dtd.nlm.nih.gov/publishing/tag-library/n-4zh0.html>).
      * @param null|mixed $contextId
      */
-    public function getByPubId(string $pubIdType, string $pubId, int $contextId = null): ?Submission
+    public function getByPubId(string $pubIdType, string $pubId, ?int $contextId = null): ?Submission
     {
         // Add check for incoming DOI request for legacy calls that bypass the Submission Repository
         if ($pubIdType == 'doi') {