diff --git a/client/includes/Hooks/BeforePageDisplayHandler.php b/client/includes/Hooks/BeforePageDisplayHandler.php
index c4c9d6db61a..77266d08cab 100644
--- a/client/includes/Hooks/BeforePageDisplayHandler.php
+++ b/client/includes/Hooks/BeforePageDisplayHandler.php
@@ -80,7 +80,7 @@ private function addStyleModules( OutputPage $outputPage, Title $title, string $
 		}
 	}
 
-	private function addJsModules( OutputPage $outputPage, Title $title, $actionName, Skin $skin ): void {
+	private function addJsModules( OutputPage $outputPage, Title $title, string $actionName, Skin $skin ): void {
 		$user = $outputPage->getUser();
 
 		if ( $this->hasLinkItemWidget( $user, $outputPage, $title, $actionName ) ) {
diff --git a/client/includes/Hooks/SiteLinksForDisplayLookup.php b/client/includes/Hooks/SiteLinksForDisplayLookup.php
index c96157580a3..998cb793b3b 100644
--- a/client/includes/Hooks/SiteLinksForDisplayLookup.php
+++ b/client/includes/Hooks/SiteLinksForDisplayLookup.php
@@ -104,7 +104,7 @@ public function getSiteLinksForItemId( ItemId $itemId ) {
 		return $this->getSiteLinksForItem( $item );
 	}
 
-	private function getSiteLinksForItem( Item $item ) {
+	private function getSiteLinksForItem( Item $item ): array {
 		$siteLinks = $item->getSiteLinkList()->toArray();
 
 		$this->hookContainer->run( 'WikibaseClientSiteLinksForItem', [
diff --git a/client/includes/RecentChanges/ChangeLineFormatter.php b/client/includes/RecentChanges/ChangeLineFormatter.php
index a33295f5a81..bd4251d4960 100644
--- a/client/includes/RecentChanges/ChangeLineFormatter.php
+++ b/client/includes/RecentChanges/ChangeLineFormatter.php
@@ -324,7 +324,7 @@ private function buildPermanentLinkHTML( EntityId $entityId, RevisionData $rev,
 		);
 	}
 
-	private function buildDiffLinkHTML( EntityId $entityId, RevisionData $rev, $count ): string {
+	private function buildDiffLinkHTML( EntityId $entityId, RevisionData $rev, int $count ): string {
 		$params = [
 			'title' => $this->repoLinker->getEntityTitle( $entityId ),
 			'curid' => $rev->getPageId(),
diff --git a/client/includes/Specials/SpecialPagesWithBadges.php b/client/includes/Specials/SpecialPagesWithBadges.php
index 3c5ab964f71..0d79d7809ee 100644
--- a/client/includes/Specials/SpecialPagesWithBadges.php
+++ b/client/includes/Specials/SpecialPagesWithBadges.php
@@ -87,7 +87,7 @@ public function execute( $subPage ) {
 		}
 	}
 
-	private function prepareParams( $subPage ) {
+	private function prepareParams( ?string $subPage ) {
 		$badge = $this->getRequest()->getText( 'badge', $subPage ?: '' );
 
 		try {
@@ -141,7 +141,7 @@ public function getPageHeader() {
 			->getHTML( '' );
 	}
 
-	private function getOptionsArray() {
+	private function getOptionsArray(): array {
 		/** @var ItemId[] $badgeItemIds */
 		$badgeItemIds = array_map(
 			function( $badgeId ) {
diff --git a/client/tests/phpunit/integration/includes/Usage/UsageLookupContractTester.php b/client/tests/phpunit/integration/includes/Usage/UsageLookupContractTester.php
index 97d63a1025f..0f26581cca5 100644
--- a/client/tests/phpunit/integration/includes/Usage/UsageLookupContractTester.php
+++ b/client/tests/phpunit/integration/includes/Usage/UsageLookupContractTester.php
@@ -44,7 +44,7 @@ public function __construct( UsageLookup $lookup, $putUsagesCallback ) {
 		$this->putUsagesCallback = $putUsagesCallback;
 	}
 
-	private function putUsages( $pageId, array $usages ) {
+	private function putUsages( int $pageId, array $usages ) {
 		call_user_func( $this->putUsagesCallback, $pageId, $usages );
 	}
 
diff --git a/client/tests/phpunit/integration/includes/Usage/UsageTrackerContractTester.php b/client/tests/phpunit/integration/includes/Usage/UsageTrackerContractTester.php
index 001d9077188..7b4ff601beb 100644
--- a/client/tests/phpunit/integration/includes/Usage/UsageTrackerContractTester.php
+++ b/client/tests/phpunit/integration/includes/Usage/UsageTrackerContractTester.php
@@ -52,7 +52,7 @@ private function getUsages( $pageId ) {
 		return call_user_func( $this->getUsagesCallback, $pageId );
 	}
 
-	private function getTestUsages() {
+	private function getTestUsages(): array {
 		$q3 = new ItemId( 'Q3' );
 		$q4 = new ItemId( 'Q4' );
 		$q5 = new ItemId( 'Q5' );