diff --git a/apps/accessibility/lib/Controller/ConfigController.php b/apps/accessibility/lib/Controller/ConfigController.php
index 5f2f77ed3e..b918b33f31 100644
--- a/apps/accessibility/lib/Controller/ConfigController.php
+++ b/apps/accessibility/lib/Controller/ConfigController.php
@@ -117,7 +117,7 @@ class ConfigController extends OCSController {
 			$highcontrast = [$this->accessibilityProvider->getHighContrast()];
 			$fonts = $this->accessibilityProvider->getFonts();
 
-			$availableOptions = array_map(function ($option) {
+			$availableOptions = array_map(function ($option): string {
 				return $option['id'];
 			}, array_merge($themes, $highcontrast, $fonts));
 
diff --git a/apps/dav/lib/AppInfo/Application.php b/apps/dav/lib/AppInfo/Application.php
index f1bf77c328..9bc95124c2 100644
--- a/apps/dav/lib/AppInfo/Application.php
+++ b/apps/dav/lib/AppInfo/Application.php
@@ -143,7 +143,7 @@ class Application extends App implements IBootstrap {
 			}
 		});
 
-		$birthdayListener = function ($event) use ($container) {
+		$birthdayListener = function ($event) use ($container): void {
 			if ($event instanceof GenericEvent) {
 				/** @var BirthdayService $b */
 				$b = $container->query(BirthdayService::class);
@@ -168,7 +168,7 @@ class Application extends App implements IBootstrap {
 			}
 		});
 
-		$clearPhotoCache = function ($event) use ($container) {
+		$clearPhotoCache = function ($event) use ($container): void {
 			if ($event instanceof GenericEvent) {
 				/** @var PhotoCache $p */
 				$p = $container->query(PhotoCache::class);
@@ -240,7 +240,7 @@ class Application extends App implements IBootstrap {
 			);
 		});
 
-		$listener = function (GenericEvent $event, $eventName) use ($container) {
+		$listener = function (GenericEvent $event, $eventName) use ($container): void {
 			/** @var Backend $backend */
 			$backend = $container->query(Backend::class);
 
@@ -336,7 +336,7 @@ class Application extends App implements IBootstrap {
 			}
 		);
 
-		$eventHandler = function () use ($container, $serverContainer) {
+		$eventHandler = function () use ($container, $serverContainer): void {
 			try {
 				/** @var UpdateCalendarResourcesRoomsBackgroundJob $job */
 				$job = $container->query(UpdateCalendarResourcesRoomsBackgroundJob::class);
diff --git a/apps/dav/lib/BackgroundJob/UpdateCalendarResourcesRoomsBackgroundJob.php b/apps/dav/lib/BackgroundJob/UpdateCalendarResourcesRoomsBackgroundJob.php
index e394890e0e..4ed4319d70 100644
--- a/apps/dav/lib/BackgroundJob/UpdateCalendarResourcesRoomsBackgroundJob.php
+++ b/apps/dav/lib/BackgroundJob/UpdateCalendarResourcesRoomsBackgroundJob.php
@@ -399,7 +399,7 @@ class UpdateCalendarResourcesRoomsBackgroundJob extends TimedJob {
 			->where($query->expr()->eq('backend_id', $query->createNamedParameter($backendId)));
 		$stmt = $query->execute();
 
-		return array_map(function ($row) {
+		return array_map(function ($row): string {
 			return $row['resource_id'];
 		}, $stmt->fetchAll());
 	}
diff --git a/apps/dav/lib/CalDAV/ResourceBooking/AbstractPrincipalBackend.php b/apps/dav/lib/CalDAV/ResourceBooking/AbstractPrincipalBackend.php
index 481c576081..bfe4e938a5 100644
--- a/apps/dav/lib/CalDAV/ResourceBooking/AbstractPrincipalBackend.php
+++ b/apps/dav/lib/CalDAV/ResourceBooking/AbstractPrincipalBackend.php
@@ -309,7 +309,7 @@ abstract class AbstractPrincipalBackend implements BackendInterface {
 						return $this->isAllowedToAccessResource($row, $usersGroups);
 					});
 
-					$results[] = array_map(function ($row) {
+					$results[] = array_map(function ($row): string {
 						return $row['uri'];
 					}, $filteredRows);