diff --git a/core/BackgroundJobs/BackgroundCleanupUpdaterBackupsJob.php b/core/BackgroundJobs/BackgroundCleanupUpdaterBackupsJob.php
index 44a14352cd..ee476633a9 100644
--- a/core/BackgroundJobs/BackgroundCleanupUpdaterBackupsJob.php
+++ b/core/BackgroundJobs/BackgroundCleanupUpdaterBackupsJob.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright 2018 Morris Jobke <hey@morrisjobke.de>
  *
diff --git a/core/BackgroundJobs/CheckForUserCertificates.php b/core/BackgroundJobs/CheckForUserCertificates.php
index 8b106c8ce7..d8892e03b5 100644
--- a/core/BackgroundJobs/CheckForUserCertificates.php
+++ b/core/BackgroundJobs/CheckForUserCertificates.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright 2020 Morris Jobke <hey@morrisjobke.de>
  *
@@ -49,7 +52,7 @@ class CheckForUserCertificates extends QueuedJob {
 	/**
 	 * Checks all user directories for old user uploaded certificates
 	 */
-	public function run($arguments) {
+	public function run($arguments): void {
 		$uploadList = [];
 		$this->userManager->callForSeenUsers(function (IUser $user) use (&$uploadList) {
 			$userId = $user->getUID();
diff --git a/core/BackgroundJobs/CleanupLoginFlowV2.php b/core/BackgroundJobs/CleanupLoginFlowV2.php
index 9fee3550c8..69e7fe7195 100644
--- a/core/BackgroundJobs/CleanupLoginFlowV2.php
+++ b/core/BackgroundJobs/CleanupLoginFlowV2.php
@@ -42,7 +42,7 @@ class CleanupLoginFlowV2 extends TimedJob {
 		$this->setInterval(3600);
 	}
 
-	protected function run($argument) {
+	protected function run($argument): void {
 		$this->loginFlowV2Mapper->cleanup();
 	}
 }
diff --git a/core/register_command.php b/core/register_command.php
index 8741d8d1ab..c9e8605219 100644
--- a/core/register_command.php
+++ b/core/register_command.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
diff --git a/core/routes.php b/core/routes.php
index 5bc4075315..791fe8c74f 100644
--- a/core/routes.php
+++ b/core/routes.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
diff --git a/core/strings.php b/core/strings.php
index f59b5c8265..fa23fb1ac6 100644
--- a/core/strings.php
+++ b/core/strings.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
diff --git a/lib/autoloader.php b/lib/autoloader.php
index cf25f4498a..b37e20c237 100644
--- a/lib/autoloader.php
+++ b/lib/autoloader.php
@@ -68,7 +68,7 @@ class Autoloader {
 	 *
 	 * @param string $root
 	 */
-	public function addValidRoot(string $root) {
+	public function addValidRoot(string $root): void {
 		$root = stream_resolve_include_path($root);
 		$this->validRoots[$root] = true;
 	}
@@ -76,14 +76,14 @@ class Autoloader {
 	/**
 	 * disable the usage of the global classpath \OC::$CLASSPATH
 	 */
-	public function disableGlobalClassPath() {
+	public function disableGlobalClassPath(): void {
 		$this->useGlobalClassPath = false;
 	}
 
 	/**
 	 * enable the usage of the global classpath \OC::$CLASSPATH
 	 */
-	public function enableGlobalClassPath() {
+	public function enableGlobalClassPath(): void {
 		$this->useGlobalClassPath = true;
 	}
 
@@ -184,7 +184,7 @@ class Autoloader {
 	 *
 	 * @param \OC\Memcache\Cache $memoryCache Instance of memory cache.
 	 */
-	public function setMemoryCache(\OC\Memcache\Cache $memoryCache = null) {
+	public function setMemoryCache(\OC\Memcache\Cache $memoryCache = null): void {
 		$this->memoryCache = $memoryCache;
 	}
 }
diff --git a/lib/base.php b/lib/base.php
index 76d35ba754..b433806ac3 100644
--- a/lib/base.php
+++ b/lib/base.php
@@ -404,7 +404,7 @@ class OC {
 
 	public static function initSession() {
 		if (self::$server->getRequest()->getServerProtocol() === 'https') {
-			ini_set('session.cookie_secure', true);
+			ini_set('session.cookie_secure', 'true');
 		}
 
 		// prevents javascript from accessing php session cookies
diff --git a/lib/private/Updater.php b/lib/private/Updater.php
index 964fd2ad10..8d21161b1f 100644
--- a/lib/private/Updater.php
+++ b/lib/private/Updater.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  * @copyright Copyright (c) 2016, Lukas Reschke <lukas@statuscode.ch>
@@ -95,7 +98,7 @@ class Updater extends BasicEmitter {
 	 *
 	 * @return bool true if the operation succeeded, false otherwise
 	 */
-	public function upgrade() {
+	public function upgrade(): bool {
 		$this->emitRepairEvents();
 		$this->logAllEvents();
 
@@ -160,7 +163,7 @@ class Updater extends BasicEmitter {
 	 *
 	 * @return array allowed previous versions per vendor
 	 */
-	private function getAllowedPreviousVersions() {
+	private function getAllowedPreviousVersions(): array {
 		// this should really be a JSON file
 		require \OC::$SERVERROOT . '/version.php';
 		/** @var array $OC_VersionCanBeUpgradedFrom */
@@ -172,7 +175,7 @@ class Updater extends BasicEmitter {
 	 *
 	 * @return string Get the vendor
 	 */
-	private function getVendor() {
+	private function getVendor(): string {
 		// this should really be a JSON file
 		require \OC::$SERVERROOT . '/version.php';
 		/** @var string $vendor */
@@ -186,7 +189,7 @@ class Updater extends BasicEmitter {
 	 * @param array $allowedPreviousVersions
 	 * @return bool
 	 */
-	public function isUpgradePossible($oldVersion, $newVersion, array $allowedPreviousVersions) {
+	public function isUpgradePossible(string $oldVersion, string $newVersion, array $allowedPreviousVersions): bool {
 		$version = explode('.', $oldVersion);
 		$majorMinor = $version[0] . '.' . $version[1];
 
@@ -221,7 +224,7 @@ class Updater extends BasicEmitter {
 	 *
 	 * @throws \Exception
 	 */
-	private function doUpgrade($currentVersion, $installedVersion) {
+	private function doUpgrade(string $currentVersion, string $installedVersion): void {
 		// Stop update if the update is over several major versions
 		$allowedPreviousVersions = $this->getAllowedPreviousVersions();
 		if (!$this->isUpgradePossible($installedVersion, $currentVersion, $allowedPreviousVersions)) {
@@ -296,7 +299,7 @@ class Updater extends BasicEmitter {
 		$this->config->setAppValue('core', 'vendor', $this->getVendor());
 	}
 
-	protected function doCoreUpgrade() {
+	protected function doCoreUpgrade(): void {
 		$this->emit('\OC\Updater', 'dbUpgradeBefore');
 
 		// execute core migrations
@@ -312,7 +315,7 @@ class Updater extends BasicEmitter {
 	 *
 	 * @throws NeedsUpdateException
 	 */
-	protected function doAppUpgrade() {
+	protected function doAppUpgrade(): void {
 		$apps = \OC_App::getEnabledApps();
 		$priorityTypes = ['authentication', 'filesystem', 'logging'];
 		$pseudoOtherType = 'other';
@@ -361,7 +364,7 @@ class Updater extends BasicEmitter {
 	 * @return array
 	 * @throws \Exception
 	 */
-	private function checkAppsRequirements() {
+	private function checkAppsRequirements(): array {
 		$isCoreUpgrade = $this->isCodeUpgrade();
 		$apps = OC_App::getEnabledApps();
 		$version = implode('.', Util::getVersion());
@@ -396,7 +399,7 @@ class Updater extends BasicEmitter {
 	/**
 	 * @return bool
 	 */
-	private function isCodeUpgrade() {
+	private function isCodeUpgrade(): bool {
 		$installedVersion = $this->config->getSystemValue('version', '0.0.0');
 		$currentVersion = implode('.', Util::getVersion());
 		if (version_compare($currentVersion, $installedVersion, '>')) {
@@ -410,7 +413,7 @@ class Updater extends BasicEmitter {
 	 * @param bool $reenable
 	 * @throws \Exception
 	 */
-	private function upgradeAppStoreApps(array $disabledApps, $reenable = false) {
+	private function upgradeAppStoreApps(array $disabledApps, bool $reenable = false): void {
 		foreach ($disabledApps as $app) {
 			try {
 				$this->emit('\OC\Updater', 'checkAppStoreAppBefore', [$app]);
@@ -435,7 +438,7 @@ class Updater extends BasicEmitter {
 	/**
 	 * Forward messages emitted by the repair routine
 	 */
-	private function emitRepairEvents() {
+	private function emitRepairEvents(): void {
 		$dispatcher = \OC::$server->getEventDispatcher();
 		$dispatcher->addListener('\OC\Repair::warning', function ($event) {
 			if ($event instanceof GenericEvent) {
@@ -459,7 +462,7 @@ class Updater extends BasicEmitter {
 		});
 	}
 
-	private function logAllEvents() {
+	private function logAllEvents(): void {
 		$log = $this->log;
 
 		$dispatcher = \OC::$server->getEventDispatcher();
diff --git a/lib/public/AutoloadNotAllowedException.php b/lib/public/AutoloadNotAllowedException.php
index 9d41315c32..6608e1739c 100644
--- a/lib/public/AutoloadNotAllowedException.php
+++ b/lib/public/AutoloadNotAllowedException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
diff --git a/lib/public/Constants.php b/lib/public/Constants.php
index 86e1fb6c53..813fbe673d 100644
--- a/lib/public/Constants.php
+++ b/lib/public/Constants.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
diff --git a/lib/public/Defaults.php b/lib/public/Defaults.php
index 8f5ed68d18..3173334664 100644
--- a/lib/public/Defaults.php
+++ b/lib/public/Defaults.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
@@ -65,7 +68,7 @@ class Defaults {
 	 * @return string
 	 * @since 6.0.0
 	 */
-	public function getBaseUrl() {
+	public function getBaseUrl(): string {
 		return $this->defaults->getBaseUrl();
 	}
 
@@ -74,7 +77,7 @@ class Defaults {
 	 * @return string
 	 * @since 6.0.0
 	 */
-	public function getSyncClientUrl() {
+	public function getSyncClientUrl(): string {
 		return $this->defaults->getSyncClientUrl();
 	}
 
@@ -83,7 +86,7 @@ class Defaults {
 	 * @return string
 	 * @since 8.0.0
 	 */
-	public function getiOSClientUrl() {
+	public function getiOSClientUrl(): string {
 		return $this->defaults->getiOSClientUrl();
 	}
 
@@ -92,7 +95,7 @@ class Defaults {
 	 * @return string
 	 * @since 8.0.0
 	 */
-	public function getAndroidClientUrl() {
+	public function getAndroidClientUrl(): string {
 		return $this->defaults->getAndroidClientUrl();
 	}
 
@@ -101,7 +104,7 @@ class Defaults {
 	 * @return string
 	 * @since 6.0.0
 	 */
-	public function getDocBaseUrl() {
+	public function getDocBaseUrl(): string {
 		return $this->defaults->getDocBaseUrl();
 	}
 
@@ -110,7 +113,7 @@ class Defaults {
 	 * @return string
 	 * @since 6.0.0
 	 */
-	public function getName() {
+	public function getName(): string {
 		return $this->defaults->getName();
 	}
 
@@ -120,7 +123,7 @@ class Defaults {
 	 * @since 8.0.0
 	 * @depreacted 22.0.0
 	 */
-	public function getHTMLName() {
+	public function getHTMLName(): string {
 		return $this->defaults->getHTMLName();
 	}
 
@@ -129,7 +132,7 @@ class Defaults {
 	 * @return string
 	 * @since 6.0.0
 	 */
-	public function getEntity() {
+	public function getEntity(): string {
 		return $this->defaults->getEntity();
 	}
 
@@ -138,7 +141,7 @@ class Defaults {
 	 * @return string
 	 * @since 6.0.0
 	 */
-	public function getSlogan(?string $lang = null) {
+	public function getSlogan(?string $lang = null): string {
 		return $this->defaults->getSlogan($lang);
 	}
 
@@ -147,7 +150,7 @@ class Defaults {
 	 * @return string
 	 * @since 6.0.0
 	 */
-	public function getShortFooter() {
+	public function getShortFooter(): string {
 		return $this->defaults->getShortFooter();
 	}
 
@@ -156,7 +159,7 @@ class Defaults {
 	 * @return string
 	 * @since 6.0.0
 	 */
-	public function getLongFooter() {
+	public function getLongFooter(): string {
 		return $this->defaults->getLongFooter();
 	}
 
@@ -165,7 +168,7 @@ class Defaults {
 	 * @return string AppId
 	 * @since 8.0.0
 	 */
-	public function getiTunesAppId() {
+	public function getiTunesAppId(): string {
 		return $this->defaults->getiTunesAppId();
 	}
 
@@ -176,7 +179,7 @@ class Defaults {
 	 * @return string
 	 * @since 12.0.0
 	 */
-	public function getLogo($useSvg = true) {
+	public function getLogo(bool $useSvg = true): string {
 		return $this->defaults->getLogo($useSvg);
 	}
 
@@ -185,7 +188,7 @@ class Defaults {
 	 * @return string
 	 * @since 12.0.0
 	 */
-	public function getColorPrimary() {
+	public function getColorPrimary(): string {
 		return $this->defaults->getColorPrimary();
 	}
 
@@ -194,7 +197,7 @@ class Defaults {
 	 * @return string URL to doc with key
 	 * @since 12.0.0
 	 */
-	public function buildDocLinkToKey($key) {
+	public function buildDocLinkToKey(string $key): string {
 		return $this->defaults->buildDocLinkToKey($key);
 	}
 
@@ -203,7 +206,7 @@ class Defaults {
 	 * @return string title
 	 * @since 12.0.0
 	 */
-	public function getTitle() {
+	public function getTitle(): string {
 		return $this->defaults->getTitle();
 	}
 
@@ -212,7 +215,7 @@ class Defaults {
 	 * @return string
 	 * @since 13.0.0
 	 */
-	public function getTextColorPrimary() {
+	public function getTextColorPrimary(): string {
 		return $this->defaults->getTextColorPrimary();
 	}
 }
diff --git a/lib/public/PreConditionNotMetException.php b/lib/public/PreConditionNotMetException.php
index 1871a16088..f48ac01ade 100644
--- a/lib/public/PreConditionNotMetException.php
+++ b/lib/public/PreConditionNotMetException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
diff --git a/lib/versioncheck.php b/lib/versioncheck.php
index efd4e0f377..83ee705a77 100644
--- a/lib/versioncheck.php
+++ b/lib/versioncheck.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  *
  *
diff --git a/tests/docker/mysqlmb4.config.php b/tests/docker/mysqlmb4.config.php
index 528673950a..3511e3c372 100644
--- a/tests/docker/mysqlmb4.config.php
+++ b/tests/docker/mysqlmb4.config.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 $CONFIG = [
 	'mysql.utf8mb4' => true,
 ];