diff --git a/classes/filter/ClassTypeDescription.php b/classes/filter/ClassTypeDescription.php
index de81feb76e4..9988f4aab9d 100644
--- a/classes/filter/ClassTypeDescription.php
+++ b/classes/filter/ClassTypeDescription.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * @file classes/filter/ClassTypeDescription.php
  *
@@ -20,10 +21,10 @@
 class ClassTypeDescription extends TypeDescription
 {
     /** @var string a valid class name */
-    public $_className;
+    public string $_className;
 
     /** @var string a valid package name */
-    public $_packageName;
+    public string $_packageName;
 
     //
     // Setters and Getters
@@ -43,7 +44,7 @@ public function getNamespace()
     /**
      * @see TypeDescription::parseTypeName()
      */
-    public function parseTypeName($typeName)
+    public function parseTypeName(string $typeName): bool
     {
         $splitName = $this->splitClassName($typeName);
         if ($splitName === false) {
@@ -61,7 +62,7 @@ public function parseTypeName($typeName)
     /**
      * @see TypeDescription::checkType()
      */
-    public function checkType($object)
+    public function checkType($object): bool
     {
         // We expect an object
         if (!is_object($object)) {
@@ -93,6 +94,7 @@ public function checkType($object)
     public function splitClassName($typeName)
     {
         // This should be a class - identify package and class name
+        // This behaviour is DEPRECATED with pkp/pkp-lib#8186
         $typeNameParts = explode('.', $typeName);
 
         $className = array_pop($typeNameParts);
diff --git a/classes/filter/Filter.php b/classes/filter/Filter.php
index 4d6676ae38d..eb8cff2f493 100644
--- a/classes/filter/Filter.php
+++ b/classes/filter/Filter.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * @file classes/filter/Filter.php
  *
@@ -187,7 +188,7 @@ public function setTransformationType(TypeDescription|string &$inputType, TypeDe
     /**
      * Get the input type
      */
-    public function &getInputType(): TypeDescription
+    public function getInputType(): TypeDescription
     {
         return $this->_inputType;
     }
@@ -195,7 +196,7 @@ public function &getInputType(): TypeDescription
     /**
      * Get the output type
      */
-    public function &getOutputType(): TypeDescription
+    public function getOutputType(): TypeDescription
     {
         return $this->_outputType;
     }
@@ -278,9 +279,9 @@ public function clearErrors(): void
      * Set the required runtime environment
      *
      */
-    public function setRuntimeEnvironment(RuntimeEnvironment &$runtimeEnvironment): void
+    public function setRuntimeEnvironment(RuntimeEnvironment $runtimeEnvironment): void
     {
-        $this->_runtimeEnvironment = &$runtimeEnvironment;
+        $this->_runtimeEnvironment = $runtimeEnvironment;
 
         // Inject the runtime settings into the data object
         // for persistence.
@@ -294,7 +295,7 @@ public function setRuntimeEnvironment(RuntimeEnvironment &$runtimeEnvironment):
     /**
      * Get the required runtime environment
      */
-    public function &getRuntimeEnvironment(): RuntimeEnvironment
+    public function getRuntimeEnvironment(): RuntimeEnvironment
     {
         return $this->_runtimeEnvironment;
     }
@@ -339,7 +340,7 @@ abstract public function &process(&$input);
     public function supports(&$input, &$output): bool
     {
         // Validate input
-        $inputType = &$this->getInputType();
+        $inputType = $this->getInputType();
         $validInput = $inputType->isCompatible($input);
 
         // If output is null then we're done
@@ -348,7 +349,7 @@ public function supports(&$input, &$output): bool
         }
 
         // Validate output
-        $outputType = &$this->getOutputType();
+        $outputType = $this->getOutputType();
         $validOutput = $outputType->isCompatible($output);
 
         return $validInput && $validOutput;
diff --git a/classes/filter/FilterGroupDAO.php b/classes/filter/FilterGroupDAO.php
index 894b94f0cb1..f2b26bf91e0 100644
--- a/classes/filter/FilterGroupDAO.php
+++ b/classes/filter/FilterGroupDAO.php
@@ -29,7 +29,7 @@ class FilterGroupDAO extends \PKP\db\DAO
      *
      * @return int the new filter group id
      */
-    public function insertObject(FilterGroup &$filterGroup): int
+    public function insertObject(FilterGroup $filterGroup): int
     {
         $this->update(
             sprintf('INSERT INTO filter_groups
@@ -87,7 +87,7 @@ public function getObjectBySymbolic(string $filterGroupSymbolic): ?FilterGroup
     /**
      * Update an existing filter group.
      */
-    public function updateObject(FilterGroup $filterGroup)
+    public function updateObject(FilterGroup $filterGroup): void
     {
         $this->update(
             'UPDATE	filter_groups
diff --git a/classes/filter/PrimitiveTypeDescription.php b/classes/filter/PrimitiveTypeDescription.php
index 5b6ae59effb..706baed21bd 100644
--- a/classes/filter/PrimitiveTypeDescription.php
+++ b/classes/filter/PrimitiveTypeDescription.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * @file classes/filter/PrimitiveTypeDescription.php
  *
@@ -39,7 +40,7 @@ public function getNamespace()
     /**
      * @see TypeDescription::parseTypeName()
      */
-    public function parseTypeName($typeName)
+    public function parseTypeName(string $typeName): bool
     {
         // This should be a primitive type
         if (!in_array($typeName, $this->_supportedPrimitiveTypes())) {
@@ -53,7 +54,7 @@ public function parseTypeName($typeName)
     /**
      * @see TypeDescription::checkType()
      */
-    public function checkType($object)
+    public function checkType($object): bool
     {
         // We expect a primitive type
         if (!is_scalar($object)) {
diff --git a/classes/filter/TypeDescription.php b/classes/filter/TypeDescription.php
index 3147c4e4ee8..e9784fa97c5 100644
--- a/classes/filter/TypeDescription.php
+++ b/classes/filter/TypeDescription.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * @file classes/filter/TypeDescription.php
  *
@@ -179,20 +180,13 @@ public function isCompatible($object)
     //
     /**
      * Parse a type name
-     *
-     * @param string $typeName
-     *
-     * @return bool true if success, otherwise false
      */
-    abstract public function parseTypeName($typeName);
+    abstract public function parseTypeName(string $typeName): bool;
 
     /**
      * Validates an object against the internal type description.
-     *
-     *
-     * @return bool
      */
-    abstract public function checkType($object);
+    abstract public function checkType($object): bool;
 
 
     //
diff --git a/classes/filter/TypeDescriptionFactory.php b/classes/filter/TypeDescriptionFactory.php
index 10839a202e4..1f7c9385580 100644
--- a/classes/filter/TypeDescriptionFactory.php
+++ b/classes/filter/TypeDescriptionFactory.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * @file classes/filter/TypeDescriptionFactory.php
  *
@@ -80,29 +81,28 @@ public static function getInstance()
      *
      * @return TypeDescription|null if the type description is invalid.
      */
-    public function &instantiateTypeDescription($typeDescription)
+    public function instantiateTypeDescription($typeDescription): ?TypeDescription
     {
-        $nullVar = null;
-
         // Identify the namespace
         $typeDescriptionParts = explode('::', $typeDescription);
         if (count($typeDescriptionParts) != 2) {
-            return $nullVar;
+            return null;
         }
 
         // Map the namespace to a type description class
         $typeDescriptionClass = $this->_namespaceMap($typeDescriptionParts[0]);
         if (is_null($typeDescriptionClass)) {
-            return $nullVar;
+            return null;
         }
 
         // Instantiate and return the type description object
-        $typeDescriptionObject = & instantiate($typeDescriptionClass, 'TypeDescription', null, null, $typeDescriptionParts[1]);
-        if (!is_object($typeDescriptionObject)) {
-            return $nullVar;
+        if (strpos($typeDescriptionClass, '.')) {
+            // DEPRECATED: Use old instantiate function
+            return instantiate($typeDescriptionClass, 'TypeDescription', null, null, $typeDescriptionParts[1]);
+        } else {
+            // New behaviour: Use fully qualified class name
+            return new $typeDescriptionClass();
         }
-
-        return $typeDescriptionObject;
     }
 
 
@@ -114,21 +114,17 @@ public function &instantiateTypeDescription($typeDescription)
      * class name.
      *
      * FIXME: Move this map to the Application object.
-     *
-     * @param string $namespace
-     *
-     * @return string
      */
-    public function _namespaceMap($namespace)
+    public function _namespaceMap(string $namespace): ?string
     {
-        static $namespaceMap = [
+        return match($namespace) {
             self::TYPE_DESCRIPTION_NAMESPACE_PRIMITIVE => 'lib.pkp.classes.filter.PrimitiveTypeDescription',
             self::TYPE_DESCRIPTION_NAMESPACE_CLASS => 'lib.pkp.classes.filter.ClassTypeDescription',
             self::TYPE_DESCRIPTION_NAMESPACE_METADATA => 'lib.pkp.classes.metadata.MetadataTypeDescription',
             self::TYPE_DESCRIPTION_NAMESPACE_XML => 'lib.pkp.classes.xslt.XMLTypeDescription',
-            self::TYPE_DESCRIPTION_NAMESPACE_VALIDATOR => 'lib.pkp.classes.validation.ValidatorTypeDescription'
-        ];
-        return $namespaceMap[$namespace] ?? null;
+            self::TYPE_DESCRIPTION_NAMESPACE_VALIDATOR => 'lib.pkp.classes.validation.ValidatorTypeDescription',
+            default => null
+        };
     }
 }
 
diff --git a/classes/metadata/MetadataTypeDescription.php b/classes/metadata/MetadataTypeDescription.php
index c2f22fe7345..32cdafea694 100644
--- a/classes/metadata/MetadataTypeDescription.php
+++ b/classes/metadata/MetadataTypeDescription.php
@@ -1,4 +1,5 @@
 <?php
+
 /**
  * @file classes/metadata/MetadataTypeDescription.php
  *
@@ -77,7 +78,7 @@ public function getAssocType()
     /**
      * @see TypeDescription::parseTypeName()
      */
-    public function parseTypeName($typeName)
+    public function parseTypeName(string $typeName): bool
     {
         // Configure the parent class type description
         // with the expected meta-data class.
@@ -116,7 +117,7 @@ public function parseTypeName($typeName)
     /**
      * @see TypeDescription::checkType()
      */
-    public function checkType($object)
+    public function checkType($object): bool
     {
         // First of all check whether this is a
         // meta-data description at all.
diff --git a/classes/migration/upgrade/v3_5_0/FilterClassNames.php b/classes/migration/upgrade/v3_5_0/FilterClassNames.php
new file mode 100644
index 00000000000..1ca55f44bba
--- /dev/null
+++ b/classes/migration/upgrade/v3_5_0/FilterClassNames.php
@@ -0,0 +1,61 @@
+<?php
+
+/**
+ * @file classes/migration/upgrade/v3_5_0/FilterClassNames.php
+ *
+ * Copyright (c) 2014-2024 Simon Fraser University
+ * Copyright (c) 2000-2024 John Willinsky
+ * Distributed under the GNU GPL v3. For full terms see the file docs/COPYING.
+ *
+ * @class FilterClassNames
+ *
+ * @brief Update filter class names in the database to match renamed classnames/namespaces in PHP.
+ */
+
+namespace PKP\migration\upgrade\v3_5_0;
+
+use Illuminate\Support\Facades\DB;
+use PKP\migration\Migration;
+
+class FilterClassNames extends Migration
+{
+    protected array $emailTemplatesInstalled = [];
+
+    public function up(): void
+    {
+        // UserGroup became an Eloquent model and moved; use namespacing
+        DB::table('filter_groups')
+            ->where('input_type', 'class::lib.pkp.classes.security.UserGroup[]')
+            ->update(['input_type' => 'class::PKP\\userGroup\\UserGroup[]']);
+        DB::table('filter_groups')
+            ->where('output_type', 'class::lib.pkp.classes.security.UserGroup[]')
+            ->update(['output_type' => 'class::PKP\\userGroup\\UserGroup[]']);
+
+        // User class didn't move, but use namespacing
+        DB::table('filter_groups')
+            ->where('input_type', 'class::lib.pkp.classes.user.User[]')
+            ->update(['input_type' => 'class::PKP\\user\\User[]']);
+        DB::table('filter_groups')
+            ->where('output_type', 'class::classes.users.User[]')
+            ->update(['output_type' => 'class::PKP\\user\\User[]']);
+    }
+
+    public function down(): void
+    {
+        // UserGroup
+        DB::table('filter_groups')
+            ->where('input_type', 'class::PKP\\userGroup\\UserGroup[]')
+            ->update(['input_type' => 'class::lib.pkp.classes.security.UserGroup[]']);
+        DB::table('filter_groups')
+            ->where('output_type', 'class::PKP\\userGroup\\UserGroup[]')
+            ->update(['output_type' => 'class::lib.pkp.classes.security.UserGroup[]']);
+
+        // User
+        DB::table('filter_groups')
+            ->where('input_type', 'class::PKP\\user\\User[]')
+            ->update(['input_type' => 'class::lib.pkp.classes.user.User[]']);
+        DB::table('filter_groups')
+            ->where('output_type', 'class::PKP\\user\\User[]')
+            ->update(['output_type' => 'class::classes.users.User[]']);
+    }
+}
diff --git a/classes/validation/ValidatorTypeDescription.php b/classes/validation/ValidatorTypeDescription.php
index 3e208d8970b..0c7cc3c032a 100644
--- a/classes/validation/ValidatorTypeDescription.php
+++ b/classes/validation/ValidatorTypeDescription.php
@@ -47,7 +47,7 @@ public function getNamespace()
     /**
      * @see TypeDescription::parseTypeName()
      */
-    public function parseTypeName($typeName)
+    public function parseTypeName(string $typeName): bool
     {
         // Standard validators are based on string input.
         parent::parseTypeName('string');
@@ -85,7 +85,7 @@ public function parseTypeName($typeName)
     /**
      * @see TypeDescription::checkType()
      */
-    public function checkType($object)
+    public function checkType($object): bool
     {
         // Check primitive type.
         if (!parent::checkType($object)) {
diff --git a/classes/xslt/XMLTypeDescription.php b/classes/xslt/XMLTypeDescription.php
index 1ef6330ace0..5fe48b46da8 100644
--- a/classes/xslt/XMLTypeDescription.php
+++ b/classes/xslt/XMLTypeDescription.php
@@ -69,7 +69,7 @@ public function setValidationStrategy($validationStrategy)
     /**
      * @copydoc TypeDescription::parseTypeName()
      */
-    public function parseTypeName($typeName)
+    public function parseTypeName(string $typeName): bool
     {
         // We expect a validation strategy and an optional validation argument
         $typeNameParts = explode('(', $typeName);
@@ -107,7 +107,7 @@ public function parseTypeName($typeName)
     /**
      * @copydoc TypeDescription::checkType()
      */
-    public function checkType($object)
+    public function checkType($object): bool
     {
         // We only accept DOMDocument objects and source strings.
         if (!$object instanceof DOMDocument && !is_string($object)) {
diff --git a/plugins/importexport/native/filter/NativeExportFilter.php b/plugins/importexport/native/filter/NativeExportFilter.php
index 7b922bafb97..2ed216c717e 100644
--- a/plugins/importexport/native/filter/NativeExportFilter.php
+++ b/plugins/importexport/native/filter/NativeExportFilter.php
@@ -50,23 +50,27 @@ public function getNoValidation(): ?bool
     public function supports(&$input, &$output): bool
     {
         // Validate input
-        $inputType = & $this->getInputType();
-        $validInput = $inputType->isCompatible($input);
+        $inputType = $this->getInputType();
+        if (!$inputType || !$inputType->isCompatible($input)) {
+            return false;
+        }
 
         // If output is null then we're done
         if (is_null($output)) {
-            return $validInput;
+            return true;
         }
 
         // Validate output
-        $outputType = & $this->getOutputType();
+        $outputType = $this->getOutputType();
+        if (!$outputType) {
+            return false;
+        }
 
-        if ($outputType instanceof XMLTypeDescription && $this->getNoValidation()) {
+        if ($this->getNoValidation()) {
             $outputType->setValidationStrategy(XMLTypeDescription::XML_TYPE_DESCRIPTION_VALIDATE_NONE);
         }
-        $validOutput = $outputType->isCompatible($output);
 
-        return $validInput && $validOutput;
+        return $outputType->isCompatible($output);
     }
 
     //
diff --git a/plugins/importexport/users/filter/PKPUserUserXmlFilter.php b/plugins/importexport/users/filter/PKPUserUserXmlFilter.php
index 2445df1c330..6ed3c357152 100644
--- a/plugins/importexport/users/filter/PKPUserUserXmlFilter.php
+++ b/plugins/importexport/users/filter/PKPUserUserXmlFilter.php
@@ -136,7 +136,7 @@ public function createPKPUserNode($doc, $user)
             ->get();
 
         foreach ($userGroups as $userGroup) {
-            $userNode->appendChild($doc->createElementNS($deployment->getNamespace(), 'user_group_ref', htmlspecialchars($userGroup->getName($context->getPrimaryLocale()), ENT_COMPAT, 'UTF-8')));
+            $userNode->appendChild($doc->createElementNS($deployment->getNamespace(), 'user_group_ref', htmlspecialchars($userGroup->name[$context->getPrimaryLocale()], ENT_COMPAT, 'UTF-8')));
         }
 
         // Add Reviewing Interests, if any.
@@ -152,8 +152,7 @@ public function addUserGroups($doc, $rootNode)
         $context = $deployment->getContext();
         $userGroupsNode = $doc->createElementNS($deployment->getNamespace(), 'user_groups');
 
-        $userGroups = UserGroup::withContextIds([$context->getId()])
-        ->get();
+        $userGroups = UserGroup::withContextIds([$context->getId()])->get();
 
         $filterDao = DAORegistry::getDAO('FilterDAO'); /** @var FilterDAO $filterDao */
         $userGroupExportFilters = $filterDao->getObjectsByGroup('usergroup=>user-xml');
@@ -161,8 +160,7 @@ public function addUserGroups($doc, $rootNode)
         $exportFilter = array_shift($userGroupExportFilters);
         $exportFilter->setDeployment($this->getDeployment());
 
-        $userGroupsArray = $userGroups->toArray();
-        $userGroupsDoc = $exportFilter->execute($userGroupsArray);
+        $userGroupsDoc = $exportFilter->execute($userGroups->all());
         if ($userGroupsDoc->documentElement instanceof \DOMElement) {
             $clone = $doc->importNode($userGroupsDoc->documentElement, true);
             $rootNode->appendChild($clone);
diff --git a/plugins/importexport/users/filter/UserGroupNativeXmlFilter.php b/plugins/importexport/users/filter/UserGroupNativeXmlFilter.php
index ec35792d241..73858170a65 100644
--- a/plugins/importexport/users/filter/UserGroupNativeXmlFilter.php
+++ b/plugins/importexport/users/filter/UserGroupNativeXmlFilter.php
@@ -16,7 +16,6 @@
 
 namespace PKP\plugins\importexport\users\filter;
 
-use APP\facades\Repo;
 use DOMDocument;
 use PKP\filter\FilterGroup;
 use PKP\userGroup\UserGroup;
@@ -86,8 +85,8 @@ public function createUserGroupNode($doc, $userGroup)
         $userGroupNode->appendChild($doc->createElementNS($deployment->getNamespace(), 'context_id', $userGroup->contextId));
         $userGroupNode->appendChild($doc->createElementNS($deployment->getNamespace(), 'is_default', $userGroup->isDefault ? 'true' : 'false'));
         $userGroupNode->appendChild($doc->createElementNS($deployment->getNamespace(), 'show_title', $userGroup->showTitle ? 'true' : 'false'));
-        $userGroupNode->appendChild($doc->createElementNS($deployment->getNamespace(), 'permit_selfRegistration', $userGroup->permitSelfRegistration ? 'true' : 'false'));
-        $userGroupNode->appendChild($doc->createElementNS($deployment->getNamespace(), 'permit_metadataEdit', $userGroup->permitMetadataEdit ? 'true' : 'false'));
+        $userGroupNode->appendChild($doc->createElementNS($deployment->getNamespace(), 'permit_self_registration', $userGroup->permitSelfRegistration ? 'true' : 'false'));
+        $userGroupNode->appendChild($doc->createElementNS($deployment->getNamespace(), 'permit_metadata_edit', $userGroup->permitMetadataEdit ? 'true' : 'false'));
 
         $this->createLocalizedNodes($doc, $userGroupNode, 'name', $userGroup->name);
         $this->createLocalizedNodes($doc, $userGroupNode, 'abbrev', $userGroup->abbrev);
diff --git a/plugins/importexport/users/filter/UserXmlPKPUserFilter.php b/plugins/importexport/users/filter/UserXmlPKPUserFilter.php
index f7092a47312..df451d2d46a 100644
--- a/plugins/importexport/users/filter/UserXmlPKPUserFilter.php
+++ b/plugins/importexport/users/filter/UserXmlPKPUserFilter.php
@@ -306,9 +306,9 @@ public function parseUser($node)
                         // if the given user associated group name in within tag 'user_group_ref' is in the list of $userGroup name local list
                         // and the user is not already assigned to that group
                         if (in_array($n->textContent, $userGroup->name) &&
-                            !UserGroup::userInGroup($userId, $userGroup->id)) {
+                            !Repo::userGroup()->userInGroup($userId, $userGroup->id)) {
                             // Found a candidate; assign user to it.
-                            UserGroup::assignUserToGroup($userId, $userGroup->id);
+                            Repo::userGroup()->assignUserToGroup($userId, $userGroup->id);
                         }
                     }
                 }
diff --git a/plugins/importexport/users/filter/filterConfig.xml b/plugins/importexport/users/filter/filterConfig.xml
index 79c218a75c1..57e1281a987 100644
--- a/plugins/importexport/users/filter/filterConfig.xml
+++ b/plugins/importexport/users/filter/filterConfig.xml
@@ -16,7 +16,7 @@
 			symbolic="user=>user-xml"
 			displayName="plugins.importexport.users.displayName"
 			description="plugins.importexport.users.description"
-			inputType="class::lib.pkp.classes.user.User[]"
+			inputType="class::PKP\user\User[]"
 			outputType="xml::schema(lib/pkp/plugins/importexport/users/pkp-users.xsd)" />
 		<!-- Native XML Users input -->
 		<filterGroup
@@ -24,13 +24,13 @@
 			displayName="plugins.importexport.users.displayName"
 			description="plugins.importexport.users.description"
 			inputType="xml::schema(lib/pkp/plugins/importexport/users/pkp-users.xsd)"
-			outputType="class::classes.users.User[]" />
+			outputType="class::PKP\user\User[]" />
 		<!-- Native XML usergroup output -->
 		<filterGroup
 			symbolic="usergroup=>user-xml"
 			displayName="plugins.importexport.users.displayName"
 			description="plugins.importexport.users.description"
-			inputType="class::lib.pkp.classes.security.UserGroup[]"
+			inputType="class::PKP\userGroup\UserGroup[]"
 			outputType="xml::schema(lib/pkp/plugins/importexport/users/pkp-users.xsd)" />
 		<!-- Native XML usergroup input -->
 		<filterGroup
@@ -38,7 +38,7 @@
 			displayName="plugins.importexport.native.displayName"
 			description="plugins.importexport.native.description"
 			inputType="xml::schema(lib/pkp/plugins/importexport/users/pkp-users.xsd)"
-			outputType="class::lib.pkp.classes.security.UserGroup[]" />
+			outputType="class::PKP\userGroup\UserGroup[]" />
 	</filterGroups>
 	<filters>
 		<!-- Native XML users output -->