diff --git a/tests/lib/Comments/ManagerTest.php b/tests/lib/Comments/ManagerTest.php
index d0cb1e00fe891..762933fb254ce 100644
--- a/tests/lib/Comments/ManagerTest.php
+++ b/tests/lib/Comments/ManagerTest.php
@@ -54,7 +54,14 @@ protected function setUp(): void {
 		$this->connection->prepare($sql)->execute();
 	}
 
-	protected function addDatabaseEntry(?string $parentId, ?string $topmostParentId, ?\DateTimeInterface $creationDT = null, ?\DateTimeInterface $latestChildDT = null, $objectId = null, ?\DateTimeInterface $expireDate = null): string {
+	protected function addDatabaseEntry(
+		?string $parentId,
+		?string $topmostParentId,
+		?\DateTimeInterface $creationDT = null,
+		?\DateTimeInterface $latestChildDT = null,
+		?string $objectId = null,
+		?\DateTimeInterface $expireDate = null
+	): string {
 		$creationDT ??= new \DateTime();
 		$latestChildDT ??= new \DateTime('yesterday');
 		$objectId ??= 'file64';
@@ -353,11 +360,11 @@ public function testGetNumberOfUnreadCommentsForFolder(): void {
 		// 2 comments for 1112 with no read marker
 		// 1 comment for 1113 before read marker
 		// 1 comment for 1114 with no read marker
-		$this->addDatabaseEntry('0', '0', null, null, $fileIds[1]);
+		$this->addDatabaseEntry('0', '0', null, null, (string)$fileIds[1]);
 		for ($i = 0; $i < 4; $i++) {
-			$this->addDatabaseEntry('0', '0', null, null, $fileIds[$i]);
+			$this->addDatabaseEntry('0', '0', null, null, (string)$fileIds[$i]);
 		}
-		$this->addDatabaseEntry('0', '0', (new \DateTime())->modify('-2 days'), null, $fileIds[0]);
+		$this->addDatabaseEntry('0', '0', (new \DateTime())->modify('-2 days'), null, (string)$fileIds[0]);
 		/** @var IUser|\PHPUnit\Framework\MockObject\MockObject $user */
 		$user = $this->createMock(IUser::class);
 		$user->expects($this->any())
diff --git a/tests/lib/User/Backend.php b/tests/lib/User/Backend.php
index dc5b245fa0678..0a1929be59672 100644
--- a/tests/lib/User/Backend.php
+++ b/tests/lib/User/Backend.php
@@ -86,10 +86,10 @@ public function testLogin(): void {
 	}
 
 	public function testSearch(): void {
-		$name1 = 'foobarbaz';
-		$name2 = 'bazbarfoo';
-		$name3 = 'notme';
-		$name4 = 'under_score';
+		$name1 = $this->getUser() . 'foobarbaz';
+		$name2 = $this->getUser() . 'bazbarfoo';
+		$name3 = $this->getUser() . 'notme';
+		$name4 = $this->getUser() . 'under_score';
 
 		$this->backend->createUser($name1, 'pass1');
 		$this->backend->createUser($name2, 'pass2');
diff --git a/tests/lib/User/DatabaseTest.php b/tests/lib/User/DatabaseTest.php
index 0afd54c345c41..e5dbcb8936d39 100644
--- a/tests/lib/User/DatabaseTest.php
+++ b/tests/lib/User/DatabaseTest.php
@@ -93,14 +93,14 @@ function (Event $event): void {
 	}
 
 	public function testCreateUserInvalidatesCache(): void {
-		$user1 = $this->getUniqueID('test_');
+		$user1 = $this->getUser();
 		$this->assertFalse($this->backend->userExists($user1));
 		$this->backend->createUser($user1, 'pw');
 		$this->assertTrue($this->backend->userExists($user1));
 	}
 
 	public function testDeleteUserInvalidatesCache(): void {
-		$user1 = $this->getUniqueID('test_');
+		$user1 = $this->getUser();
 		$this->backend->createUser($user1, 'pw');
 		$this->assertTrue($this->backend->userExists($user1));
 		$this->backend->deleteUser($user1);