diff --git a/apps/workflowengine/lib/Check/FileSize.php b/apps/workflowengine/lib/Check/FileSize.php
index 1744793dec..7e48f0f603 100644
--- a/apps/workflowengine/lib/Check/FileSize.php
+++ b/apps/workflowengine/lib/Check/FileSize.php
@@ -103,13 +103,13 @@ class FileSize implements ICheck {
 		}
 
 		$size = $this->request->getHeader('OC-Total-Length');
-		if ($size === null) {
+		if ($size === '') {
 			if (in_array($this->request->getMethod(), ['POST', 'PUT'])) {
 				$size = $this->request->getHeader('Content-Length');
 			}
 		}
 
-		if ($size === null) {
+		if ($size === '') {
 			$size = false;
 		}
 
diff --git a/core/Controller/ClientFlowLoginController.php b/core/Controller/ClientFlowLoginController.php
index 0e7fbf892b..23bd42a0f1 100644
--- a/core/Controller/ClientFlowLoginController.php
+++ b/core/Controller/ClientFlowLoginController.php
@@ -115,7 +115,7 @@ class ClientFlowLoginController extends Controller {
 	 */
 	private function getClientName() {
 		$userAgent = $this->request->getHeader('USER_AGENT');
-		return $userAgent !== null ? $userAgent : 'unknown';
+		return $userAgent !== '' ? $userAgent : 'unknown';
 	}
 
 	/**
diff --git a/core/Controller/CssController.php b/core/Controller/CssController.php
index 95a41f8dd1..43a4f453b0 100644
--- a/core/Controller/CssController.php
+++ b/core/Controller/CssController.php
@@ -98,7 +98,7 @@ class CssController extends Controller {
 	private function getFile(ISimpleFolder $folder, $fileName, &$gzip) {
 		$encoding = $this->request->getHeader('Accept-Encoding');
 
-		if ($encoding !== null && strpos($encoding, 'gzip') !== false) {
+		if (strpos($encoding, 'gzip') !== false) {
 			try {
 				$gzip = true;
 				return $folder->getFile($fileName . '.gzip'); # Safari doesn't like .gz
diff --git a/core/Controller/JsController.php b/core/Controller/JsController.php
index f02948e47a..670ca99725 100644
--- a/core/Controller/JsController.php
+++ b/core/Controller/JsController.php
@@ -96,7 +96,7 @@ class JsController extends Controller {
 	private function getFile(ISimpleFolder $folder, $fileName, &$gzip) {
 		$encoding = $this->request->getHeader('Accept-Encoding');
 
-		if ($encoding !== null && strpos($encoding, 'gzip') !== false) {
+		if (strpos($encoding, 'gzip') !== false) {
 			try {
 				$gzip = true;
 				return $folder->getFile($fileName . '.gzip'); # Safari doesn't like .gz
diff --git a/lib/private/AppFramework/Http/Request.php b/lib/private/AppFramework/Http/Request.php
index 77ecb02165..975c4255d5 100644
--- a/lib/private/AppFramework/Http/Request.php
+++ b/lib/private/AppFramework/Http/Request.php
@@ -324,7 +324,7 @@ class Request implements \ArrayAccess, \Countable, IRequest {
 
 		}
 
-		return null;
+		return '';
 	}
 
 	/**
@@ -404,8 +404,7 @@ class Request implements \ArrayAccess, \Countable, IRequest {
 	protected function getContent() {
 		// If the content can't be parsed into an array then return a stream resource.
 		if ($this->method === 'PUT'
-			&& $this->getHeader('Content-Length') !== 0
-			&& $this->getHeader('Content-Length') !== null
+			&& $this->getHeader('Content-Length') !== '0'
 			&& $this->getHeader('Content-Length') !== ''
 			&& strpos($this->getHeader('Content-Type'), 'application/x-www-form-urlencoded') === false
 			&& strpos($this->getHeader('Content-Type'), 'application/json') === false
diff --git a/lib/private/L10N/Factory.php b/lib/private/L10N/Factory.php
index e277a00e65..8268be05d4 100644
--- a/lib/private/L10N/Factory.php
+++ b/lib/private/L10N/Factory.php
@@ -247,7 +247,7 @@ class Factory implements IFactory {
 	 */
 	private function getLanguageFromRequest($app) {
 		$header = $this->request->getHeader('ACCEPT_LANGUAGE');
-		if ($header) {
+		if ($header !== '') {
 			$available = $this->findAvailableLanguages($app);
 
 			// E.g. make sure that 'de' is before 'de_DE'.
diff --git a/lib/private/Log/File.php b/lib/private/Log/File.php
index b6a208ad68..290f7897c9 100644
--- a/lib/private/Log/File.php
+++ b/lib/private/Log/File.php
@@ -105,7 +105,10 @@ class File {
 		} else {
 			$user = '--';
 		}
-		$userAgent = $request->getHeader('User-Agent') ?: '--';
+		$userAgent = $request->getHeader('User-Agent');
+		if ($userAgent === '') {
+			$userAgent = '--';
+		}
 		$version = $config->getValue('version', '');
 		$entry = compact(
 			'reqId',
diff --git a/lib/private/Memcache/APCu.php b/lib/private/Memcache/APCu.php
index 70f0d73d2d..3e96acdecb 100644
--- a/lib/private/Memcache/APCu.php
+++ b/lib/private/Memcache/APCu.php
@@ -158,8 +158,8 @@ class APCu extends Cache implements IMemcache {
 		} elseif (!\OC::$server->getIniWrapper()->getBool('apc.enable_cli') && \OC::$CLI) {
 			return false;
 		} elseif (
-				version_compare(phpversion('apc'), '4.0.6') === -1 &&
-				version_compare(phpversion('apcu'), '5.1.0') === -1
+				version_compare(phpversion('apc') ?: '0.0.0', '4.0.6') === -1 &&
+				version_compare(phpversion('apcu') ?: '0.0.0', '5.1.0') === -1
 		) {
 			return false;
 		} else {
diff --git a/tests/Core/Controller/ClientFlowLoginControllerTest.php b/tests/Core/Controller/ClientFlowLoginControllerTest.php
index 0e04853822..7fe87df026 100644
--- a/tests/Core/Controller/ClientFlowLoginControllerTest.php
+++ b/tests/Core/Controller/ClientFlowLoginControllerTest.php
@@ -431,6 +431,10 @@ class ClientFlowLoginControllerTest extends TestCase {
 			->expects($this->once())
 			->method('getServerHost')
 			->willReturn('example.com');
+		$this->request
+			->expects($this->any())
+			->method('getHeader')
+			->willReturn('');
 
 		$expected = new Http\RedirectResponse('nc://login/server:http://example.com&user:MyLoginName&password:MyGeneratedToken');
 		$this->assertEquals($expected, $this->clientFlowLoginController->generateAppPassword('MyStateToken'));
@@ -583,6 +587,10 @@ class ClientFlowLoginControllerTest extends TestCase {
 			->expects($this->once())
 			->method('getServerHost')
 			->willReturn('example.com');
+		$this->request
+			->expects($this->any())
+			->method('getHeader')
+			->willReturn('');
 
 		$expected = new Http\RedirectResponse('nc://login/server:http://example.com&user:MyLoginName&password:MyGeneratedToken');
 		$this->assertEquals($expected, $this->clientFlowLoginController->generateAppPassword('MyStateToken'));
@@ -594,6 +602,7 @@ class ClientFlowLoginControllerTest extends TestCase {
 				[
 					['X-Forwarded-Proto', 'http'],
 					['X-Forwarded-Ssl', 'off'],
+					['USER_AGENT', ''],
 				],
 				'http',
 				'http',
@@ -602,6 +611,7 @@ class ClientFlowLoginControllerTest extends TestCase {
 				[
 					['X-Forwarded-Proto', 'http'],
 					['X-Forwarded-Ssl', 'off'],
+					['USER_AGENT', ''],
 				],
 				'https',
 				'https',
@@ -610,6 +620,7 @@ class ClientFlowLoginControllerTest extends TestCase {
 				[
 					['X-Forwarded-Proto', 'https'],
 					['X-Forwarded-Ssl', 'off'],
+					['USER_AGENT', ''],
 				],
 				'http',
 				'https',
@@ -618,6 +629,7 @@ class ClientFlowLoginControllerTest extends TestCase {
 				[
 					['X-Forwarded-Proto', 'https'],
 					['X-Forwarded-Ssl', 'on'],
+					['USER_AGENT', ''],
 				],
 				'http',
 				'https',
@@ -626,6 +638,7 @@ class ClientFlowLoginControllerTest extends TestCase {
 				[
 					['X-Forwarded-Proto', 'http'],
 					['X-Forwarded-Ssl', 'on'],
+					['USER_AGENT', ''],
 				],
 				'http',
 				'https',
diff --git a/tests/lib/L10N/FactoryTest.php b/tests/lib/L10N/FactoryTest.php
index 602967b162..4f31784337 100644
--- a/tests/lib/L10N/FactoryTest.php
+++ b/tests/lib/L10N/FactoryTest.php
@@ -55,9 +55,16 @@ class FactoryTest extends TestCase {
 
 	/**
 	 * @param array $methods
+	 * @param bool $mockRequestGetHeaderMethod
 	 * @return Factory|\PHPUnit_Framework_MockObject_MockObject
 	 */
-	protected function getFactory(array $methods = []) {
+	protected function getFactory(array $methods = [], $mockRequestGetHeaderMethod = false) {
+		if ($mockRequestGetHeaderMethod) {
+			$this->request->expects($this->any())
+				->method('getHeader')
+				->willReturn('');
+		}
+
 		if (!empty($methods)) {
 			return $this->getMockBuilder(Factory::class)
 				->setConstructorArgs([
@@ -137,7 +144,7 @@ class FactoryTest extends TestCase {
 	}
 
 	public function testFindLanguageWithNotExistingRequestLanguageAndNotExistingStoredUserLanguage() {
-		$factory = $this->getFactory(['languageExists']);
+		$factory = $this->getFactory(['languageExists'], true);
 		$this->invokePrivate($factory, 'requestLanguage', ['de']);
 		$factory->expects($this->at(0))
 				->method('languageExists')
@@ -180,7 +187,7 @@ class FactoryTest extends TestCase {
 	}
 
 	public function testFindLanguageWithNotExistingRequestLanguageAndNotExistingStoredUserLanguageAndNotExistingDefault() {
-		$factory = $this->getFactory(['languageExists']);
+		$factory = $this->getFactory(['languageExists'], true);
 		$this->invokePrivate($factory, 'requestLanguage', ['de']);
 		$factory->expects($this->at(0))
 				->method('languageExists')
@@ -226,7 +233,7 @@ class FactoryTest extends TestCase {
 	}
 
 	public function testFindLanguageWithNotExistingRequestLanguageAndNotExistingStoredUserLanguageAndNotExistingDefaultAndNoAppInScope() {
-		$factory = $this->getFactory(['languageExists']);
+		$factory = $this->getFactory(['languageExists'], true);
 		$this->invokePrivate($factory, 'requestLanguage', ['de']);
 		$factory->expects($this->at(0))
 				->method('languageExists')