diff --git a/.gitattributes b/.gitattributes
index ebb928704..84c7add05 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,3 +1,4 @@
 /Tests export-ignore
 /phpunit.xml.dist export-ignore
+/.gitattributes export-ignore
 /.gitignore export-ignore
diff --git a/AcceptHeader.php b/AcceptHeader.php
index bbbd62a6d..c3c8d0c35 100644
--- a/AcceptHeader.php
+++ b/AcceptHeader.php
@@ -11,6 +11,9 @@
 
 namespace Symfony\Component\HttpFoundation;
 
+// Help opcache.preload discover always-needed symbols
+class_exists(AcceptHeaderItem::class);
+
 /**
  * Represents an Accept-* header.
  *
diff --git a/BinaryFileResponse.php b/BinaryFileResponse.php
index 64800b3fa..7bdbf4def 100644
--- a/BinaryFileResponse.php
+++ b/BinaryFileResponse.php
@@ -217,7 +217,7 @@ public function prepare(Request $request)
             }
             if ('x-accel-redirect' === strtolower($type)) {
                 // Do X-Accel-Mapping substitutions.
-                // @link http://wiki.nginx.org/X-accel#X-Accel-Redirect
+                // @link https://www.nginx.com/resources/wiki/start/topics/examples/x-accel/#x-accel-redirect
                 $parts = HeaderUtils::split($request->headers->get('X-Accel-Mapping', ''), ',=');
                 foreach ($parts as $part) {
                     list($pathPrefix, $location) = $part;
diff --git a/File/File.php b/File/File.php
index 4906588a7..c72a6d991 100644
--- a/File/File.php
+++ b/File/File.php
@@ -91,7 +91,7 @@ public function move($directory, $name = null)
         $renamed = rename($this->getPathname(), $target);
         restore_error_handler();
         if (!$renamed) {
-            throw new FileException(sprintf('Could not move the file "%s" to "%s" (%s)', $this->getPathname(), $target, strip_tags($error)));
+            throw new FileException(sprintf('Could not move the file "%s" to "%s" (%s).', $this->getPathname(), $target, strip_tags($error)));
         }
 
         @chmod($target, 0666 & ~umask());
@@ -106,10 +106,10 @@ protected function getTargetFile($directory, $name = null)
     {
         if (!is_dir($directory)) {
             if (false === @mkdir($directory, 0777, true) && !is_dir($directory)) {
-                throw new FileException(sprintf('Unable to create the "%s" directory', $directory));
+                throw new FileException(sprintf('Unable to create the "%s" directory.', $directory));
             }
         } elseif (!is_writable($directory)) {
-            throw new FileException(sprintf('Unable to write in the "%s" directory', $directory));
+            throw new FileException(sprintf('Unable to write in the "%s" directory.', $directory));
         }
 
         $target = rtrim($directory, '/\\').\DIRECTORY_SEPARATOR.(null === $name ? $this->getBasename() : $this->getName($name));
diff --git a/File/MimeType/MimeTypeGuesser.php b/File/MimeType/MimeTypeGuesser.php
index ece2109ca..65b4b25be 100644
--- a/File/MimeType/MimeTypeGuesser.php
+++ b/File/MimeType/MimeTypeGuesser.php
@@ -130,7 +130,7 @@ public function guess($path)
         }
 
         if (2 === \count($this->guessers) && !FileBinaryMimeTypeGuesser::isSupported() && !FileinfoMimeTypeGuesser::isSupported()) {
-            throw new \LogicException('Unable to guess the mime type as no guessers are available (Did you enable the php_fileinfo extension?)');
+            throw new \LogicException('Unable to guess the mime type as no guessers are available (Did you enable the php_fileinfo extension?).');
         }
 
         return null;
diff --git a/File/UploadedFile.php b/File/UploadedFile.php
index 0c67f8907..36fd1e65c 100644
--- a/File/UploadedFile.php
+++ b/File/UploadedFile.php
@@ -208,7 +208,7 @@ public function move($directory, $name = null)
             $moved = move_uploaded_file($this->getPathname(), $target);
             restore_error_handler();
             if (!$moved) {
-                throw new FileException(sprintf('Could not move the file "%s" to "%s" (%s)', $this->getPathname(), $target, strip_tags($error)));
+                throw new FileException(sprintf('Could not move the file "%s" to "%s" (%s).', $this->getPathname(), $target, strip_tags($error)));
             }
 
             @chmod($target, 0666 & ~umask());
diff --git a/HeaderBag.php b/HeaderBag.php
index 9ffe6f4fe..da794554b 100644
--- a/HeaderBag.php
+++ b/HeaderBag.php
@@ -219,7 +219,7 @@ public function getDate($key, \DateTime $default = null)
         }
 
         if (false === $date = \DateTime::createFromFormat(DATE_RFC2822, $value)) {
-            throw new \RuntimeException(sprintf('The %s HTTP header is not parseable (%s).', $key, $value));
+            throw new \RuntimeException(sprintf('The "%s" HTTP header is not parseable (%s).', $key, $value));
         }
 
         return $date;
diff --git a/LICENSE b/LICENSE
index a677f4376..9e936ec04 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,4 +1,4 @@
-Copyright (c) 2004-2019 Fabien Potencier
+Copyright (c) 2004-2020 Fabien Potencier
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
diff --git a/README.md b/README.md
index 8907f0b96..ac98f9b80 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,7 @@ specification.
 Resources
 ---------
 
-  * [Documentation](https://symfony.com/doc/current/components/http_foundation/index.html)
+  * [Documentation](https://symfony.com/doc/current/components/http_foundation.html)
   * [Contributing](https://symfony.com/doc/current/contributing/index.html)
   * [Report issues](https://github.com/symfony/symfony/issues) and
     [send Pull Requests](https://github.com/symfony/symfony/pulls)
diff --git a/RedirectResponse.php b/RedirectResponse.php
index 687bc04d9..4347f3a84 100644
--- a/RedirectResponse.php
+++ b/RedirectResponse.php
@@ -47,7 +47,7 @@ public function __construct(?string $url, int $status = 302, array $headers = []
             throw new \InvalidArgumentException(sprintf('The HTTP status code is not a redirect ("%s" given).', $status));
         }
 
-        if (301 == $status && !\array_key_exists('cache-control', array_change_key_case($headers, \CASE_LOWER))) {
+        if (301 == $status && !\array_key_exists('cache-control', array_change_key_case($headers, CASE_LOWER))) {
             $this->headers->remove('cache-control');
         }
     }
diff --git a/Request.php b/Request.php
index bb5409123..33f4efb5e 100644
--- a/Request.php
+++ b/Request.php
@@ -15,6 +15,14 @@
 use Symfony\Component\HttpFoundation\Exception\SuspiciousOperationException;
 use Symfony\Component\HttpFoundation\Session\SessionInterface;
 
+// Help opcache.preload discover always-needed symbols
+class_exists(AcceptHeader::class);
+class_exists(FileBag::class);
+class_exists(HeaderBag::class);
+class_exists(HeaderUtils::class);
+class_exists(ParameterBag::class);
+class_exists(ServerBag::class);
+
 /**
  * Request represents an HTTP request.
  *
@@ -725,7 +733,7 @@ public function getSession()
 
         if (null === $session) {
             @trigger_error(sprintf('Calling "%s()" when no session has been set is deprecated since Symfony 4.1 and will throw an exception in 5.0. Use "hasSession()" instead.', __METHOD__), E_USER_DEPRECATED);
-            // throw new \BadMethodCallException('Session has not been set');
+            // throw new \BadMethodCallException('Session has not been set.');
         }
 
         return $session;
@@ -1582,24 +1590,23 @@ public function isNoCache()
      * Gets the preferred format for the response by inspecting, in the following order:
      *   * the request format set using setRequestFormat
      *   * the values of the Accept HTTP header
-     *   * the content type of the body of the request.
+     *
+     * Note that if you use this method, you should send the "Vary: Accept" header
+     * in the response to prevent any issues with intermediary HTTP caches.
      */
     public function getPreferredFormat(?string $default = 'html'): ?string
     {
-        if (null !== $this->preferredFormat) {
+        if (null !== $this->preferredFormat || null !== $this->preferredFormat = $this->getRequestFormat(null)) {
             return $this->preferredFormat;
         }
 
-        $preferredFormat = null;
-        foreach ($this->getAcceptableContentTypes() as $contentType) {
-            if ($preferredFormat = $this->getFormat($contentType)) {
-                break;
+        foreach ($this->getAcceptableContentTypes() as $mimeType) {
+            if ($this->preferredFormat = $this->getFormat($mimeType)) {
+                return $this->preferredFormat;
             }
         }
 
-        $this->preferredFormat = $this->getRequestFormat($preferredFormat ?: $this->getContentType());
-
-        return $this->preferredFormat ?: $default;
+        return $default;
     }
 
     /**
diff --git a/Response.php b/Response.php
index c1a2e1488..b9177934a 100644
--- a/Response.php
+++ b/Response.php
@@ -11,6 +11,9 @@
 
 namespace Symfony\Component\HttpFoundation;
 
+// Help opcache.preload discover always-needed symbols
+class_exists(ResponseHeaderBag::class);
+
 /**
  * Response represents an HTTP response.
  *
@@ -267,10 +270,12 @@ public function prepare(Request $request)
             $this->setContent(null);
             $headers->remove('Content-Type');
             $headers->remove('Content-Length');
+            // prevent PHP from sending the Content-Type header based on default_mimetype
+            ini_set('default_mimetype', '');
         } else {
             // Content-type based on the Request
             if (!$headers->has('Content-Type')) {
-                $format = $request->getPreferredFormat();
+                $format = $request->getRequestFormat(null);
                 if (null !== $format && $mimeType = $request->getMimeType($format)) {
                     $headers->set('Content-Type', $mimeType);
                 }
@@ -628,7 +633,7 @@ public function isImmutable(): bool
     }
 
     /**
-     * Returns true if the response must be revalidated by caches.
+     * Returns true if the response must be revalidated by shared caches once it has become stale.
      *
      * This method indicates that the response must not be served stale by a
      * cache in any circumstance without first revalidating with the origin.
diff --git a/ResponseHeaderBag.php b/ResponseHeaderBag.php
index f5b7a27fd..e71034aba 100644
--- a/ResponseHeaderBag.php
+++ b/ResponseHeaderBag.php
@@ -252,10 +252,13 @@ public function getCookies($format = self::COOKIES_FLAT)
      * @param string $domain
      * @param bool   $secure
      * @param bool   $httpOnly
+     * @param string $sameSite
      */
-    public function clearCookie($name, $path = '/', $domain = null, $secure = false, $httpOnly = true)
+    public function clearCookie($name, $path = '/', $domain = null, $secure = false, $httpOnly = true/*, $sameSite = null*/)
     {
-        $this->setCookie(new Cookie($name, null, 1, $path, $domain, $secure, $httpOnly, false, null));
+        $sameSite = \func_num_args() > 5 ? func_get_arg(5) : null;
+
+        $this->setCookie(new Cookie($name, null, 1, $path, $domain, $secure, $httpOnly, false, $sameSite));
     }
 
     /**
@@ -276,13 +279,13 @@ public function makeDisposition($disposition, $filename, $filenameFallback = '')
      */
     protected function computeCacheControlValue()
     {
-        if (!$this->cacheControl && !$this->has('ETag') && !$this->has('Last-Modified') && !$this->has('Expires')) {
-            return 'no-cache, private';
-        }
-
         if (!$this->cacheControl) {
+            if ($this->has('Last-Modified') || $this->has('Expires')) {
+                return 'private, must-revalidate'; // allows for heuristic expiration (RFC 7234 Section 4.2.2) in the case of "Last-Modified"
+            }
+
             // conservative by default
-            return 'private, must-revalidate';
+            return 'no-cache, private';
         }
 
         $header = $this->getCacheControlHeader();
diff --git a/Session/Session.php b/Session/Session.php
index 2192c629e..b6973aaab 100644
--- a/Session/Session.php
+++ b/Session/Session.php
@@ -18,6 +18,11 @@
 use Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage;
 use Symfony\Component\HttpFoundation\Session\Storage\SessionStorageInterface;
 
+// Help opcache.preload discover always-needed symbols
+class_exists(AttributeBag::class);
+class_exists(FlashBag::class);
+class_exists(SessionBagProxy::class);
+
 /**
  * @author Fabien Potencier <fabien@symfony.com>
  * @author Drak <drak@zikula.org>
diff --git a/Session/Storage/Handler/AbstractSessionHandler.php b/Session/Storage/Handler/AbstractSessionHandler.php
index bcde59ee6..a196833cb 100644
--- a/Session/Storage/Handler/AbstractSessionHandler.php
+++ b/Session/Storage/Handler/AbstractSessionHandler.php
@@ -121,7 +121,7 @@ public function destroy($sessionId)
     {
         if (!headers_sent() && filter_var(ini_get('session.use_cookies'), FILTER_VALIDATE_BOOLEAN)) {
             if (!$this->sessionName) {
-                throw new \LogicException(sprintf('Session name cannot be empty, did you forget to call "parent::open()" in "%s"?.', \get_class($this)));
+                throw new \LogicException(sprintf('Session name cannot be empty, did you forget to call "parent::open()" in "%s"?.', static::class));
             }
             $cookie = SessionUtils::popSessionCookie($this->sessionName, $sessionId);
 
diff --git a/Session/Storage/Handler/MemcachedSessionHandler.php b/Session/Storage/Handler/MemcachedSessionHandler.php
index a399be5fd..6711e0a55 100644
--- a/Session/Storage/Handler/MemcachedSessionHandler.php
+++ b/Session/Storage/Handler/MemcachedSessionHandler.php
@@ -47,7 +47,7 @@ public function __construct(\Memcached $memcached, array $options = [])
         $this->memcached = $memcached;
 
         if ($diff = array_diff(array_keys($options), ['prefix', 'expiretime'])) {
-            throw new \InvalidArgumentException(sprintf('The following options are not supported "%s"', implode(', ', $diff)));
+            throw new \InvalidArgumentException(sprintf('The following options are not supported "%s".', implode(', ', $diff)));
         }
 
         $this->ttl = isset($options['expiretime']) ? (int) $options['expiretime'] : 86400;
diff --git a/Session/Storage/Handler/MongoDbSessionHandler.php b/Session/Storage/Handler/MongoDbSessionHandler.php
index 27e080021..6cb884778 100644
--- a/Session/Storage/Handler/MongoDbSessionHandler.php
+++ b/Session/Storage/Handler/MongoDbSessionHandler.php
@@ -66,7 +66,7 @@ class MongoDbSessionHandler extends AbstractSessionHandler
     public function __construct(\MongoDB\Client $mongo, array $options)
     {
         if (!isset($options['database']) || !isset($options['collection'])) {
-            throw new \InvalidArgumentException('You must provide the "database" and "collection" option for MongoDBSessionHandler');
+            throw new \InvalidArgumentException('You must provide the "database" and "collection" option for MongoDBSessionHandler.');
         }
 
         $this->mongo = $mongo;
diff --git a/Session/Storage/Handler/NativeFileSessionHandler.php b/Session/Storage/Handler/NativeFileSessionHandler.php
index bdfc9d819..effc9db54 100644
--- a/Session/Storage/Handler/NativeFileSessionHandler.php
+++ b/Session/Storage/Handler/NativeFileSessionHandler.php
@@ -38,7 +38,7 @@ public function __construct(string $savePath = null)
 
         if ($count = substr_count($savePath, ';')) {
             if ($count > 2) {
-                throw new \InvalidArgumentException(sprintf('Invalid argument $savePath \'%s\'', $savePath));
+                throw new \InvalidArgumentException(sprintf('Invalid argument $savePath \'%s\'.', $savePath));
             }
 
             // characters after last ';' are the path
@@ -46,7 +46,7 @@ public function __construct(string $savePath = null)
         }
 
         if ($baseDir && !is_dir($baseDir) && !@mkdir($baseDir, 0777, true) && !is_dir($baseDir)) {
-            throw new \RuntimeException(sprintf('Session Storage was not able to create directory "%s"', $baseDir));
+            throw new \RuntimeException(sprintf('Session Storage was not able to create directory "%s".', $baseDir));
         }
 
         ini_set('session.save_path', $savePath);
diff --git a/Session/Storage/Handler/PdoSessionHandler.php b/Session/Storage/Handler/PdoSessionHandler.php
index 7942e0a57..330ac7f2a 100644
--- a/Session/Storage/Handler/PdoSessionHandler.php
+++ b/Session/Storage/Handler/PdoSessionHandler.php
@@ -174,7 +174,7 @@ public function __construct($pdoOrDsn = null, array $options = [])
     {
         if ($pdoOrDsn instanceof \PDO) {
             if (\PDO::ERRMODE_EXCEPTION !== $pdoOrDsn->getAttribute(\PDO::ATTR_ERRMODE)) {
-                throw new \InvalidArgumentException(sprintf('"%s" requires PDO error mode attribute be set to throw Exceptions (i.e. $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION))', __CLASS__));
+                throw new \InvalidArgumentException(sprintf('"%s" requires PDO error mode attribute be set to throw Exceptions (i.e. $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION)).', __CLASS__));
             }
 
             $this->pdo = $pdoOrDsn;
@@ -219,7 +219,7 @@ public function createTable()
                 // - trailing space removal
                 // - case-insensitivity
                 // - language processing like é == e
-                $sql = "CREATE TABLE $this->table ($this->idCol VARBINARY(128) NOT NULL PRIMARY KEY, $this->dataCol BLOB NOT NULL, $this->lifetimeCol INTEGER UNSIGNED NOT NULL, $this->timeCol INTEGER UNSIGNED NOT NULL) COLLATE utf8_bin, ENGINE = InnoDB";
+                $sql = "CREATE TABLE $this->table ($this->idCol VARBINARY(128) NOT NULL PRIMARY KEY, $this->dataCol BLOB NOT NULL, $this->lifetimeCol INTEGER UNSIGNED NOT NULL, $this->timeCol INTEGER UNSIGNED NOT NULL) COLLATE utf8mb4_bin, ENGINE = InnoDB";
                 break;
             case 'sqlite':
                 $sql = "CREATE TABLE $this->table ($this->idCol TEXT NOT NULL PRIMARY KEY, $this->dataCol BLOB NOT NULL, $this->lifetimeCol INTEGER NOT NULL, $this->timeCol INTEGER NOT NULL)";
@@ -468,7 +468,7 @@ private function buildDsnFromUrl(string $dsnOrUrl): string
         }
 
         if (!isset($params['scheme'])) {
-            throw new \InvalidArgumentException('URLs without scheme are not supported to configure the PdoSessionHandler');
+            throw new \InvalidArgumentException('URLs without scheme are not supported to configure the PdoSessionHandler.');
         }
 
         $driverAliasMap = [
@@ -871,10 +871,10 @@ private function getMergeStatement(string $sessionId, string $data, int $maxlife
             $mergeStmt->bindParam(2, $sessionId, \PDO::PARAM_STR);
             $mergeStmt->bindParam(3, $data, \PDO::PARAM_LOB);
             $mergeStmt->bindValue(4, time() + $maxlifetime, \PDO::PARAM_INT);
-            $mergeStmt->bindValue(4, time(), \PDO::PARAM_INT);
-            $mergeStmt->bindParam(5, $data, \PDO::PARAM_LOB);
-            $mergeStmt->bindValue(6, time() + $maxlifetime, \PDO::PARAM_INT);
-            $mergeStmt->bindValue(6, time(), \PDO::PARAM_INT);
+            $mergeStmt->bindValue(5, time(), \PDO::PARAM_INT);
+            $mergeStmt->bindParam(6, $data, \PDO::PARAM_LOB);
+            $mergeStmt->bindValue(7, time() + $maxlifetime, \PDO::PARAM_INT);
+            $mergeStmt->bindValue(8, time(), \PDO::PARAM_INT);
         } else {
             $mergeStmt->bindParam(':id', $sessionId, \PDO::PARAM_STR);
             $mergeStmt->bindParam(':data', $data, \PDO::PARAM_LOB);
diff --git a/Session/Storage/Handler/RedisSessionHandler.php b/Session/Storage/Handler/RedisSessionHandler.php
index d8c1f8cb9..699d6da6f 100644
--- a/Session/Storage/Handler/RedisSessionHandler.php
+++ b/Session/Storage/Handler/RedisSessionHandler.php
@@ -54,16 +54,16 @@ public function __construct($redis, array $options = [])
             !$redis instanceof RedisProxy &&
             !$redis instanceof RedisClusterProxy
         ) {
-            throw new \InvalidArgumentException(sprintf('%s() expects parameter 1 to be Redis, RedisArray, RedisCluster or Predis\ClientInterface, %s given', __METHOD__, \is_object($redis) ? \get_class($redis) : \gettype($redis)));
+            throw new \InvalidArgumentException(sprintf('"%s()" expects parameter 1 to be Redis, RedisArray, RedisCluster or Predis\ClientInterface, "%s" given.', __METHOD__, \is_object($redis) ? \get_class($redis) : \gettype($redis)));
         }
 
         if ($diff = array_diff(array_keys($options), ['prefix', 'ttl'])) {
-            throw new \InvalidArgumentException(sprintf('The following options are not supported "%s"', implode(', ', $diff)));
+            throw new \InvalidArgumentException(sprintf('The following options are not supported "%s".', implode(', ', $diff)));
         }
 
         $this->redis = $redis;
         $this->prefix = $options['prefix'] ?? 'sf_s';
-        $this->ttl = $options['ttl'] ?? (int) ini_get('session.gc_maxlifetime');
+        $this->ttl = $options['ttl'] ?? null;
     }
 
     /**
@@ -79,7 +79,7 @@ protected function doRead($sessionId): string
      */
     protected function doWrite($sessionId, $data): bool
     {
-        $result = $this->redis->setEx($this->prefix.$sessionId, $this->ttl, $data);
+        $result = $this->redis->setEx($this->prefix.$sessionId, (int) ($this->ttl ?? ini_get('session.gc_maxlifetime')), $data);
 
         return $result && !$result instanceof ErrorInterface;
     }
@@ -115,6 +115,6 @@ public function gc($maxlifetime): bool
      */
     public function updateTimestamp($sessionId, $data)
     {
-        return (bool) $this->redis->expire($this->prefix.$sessionId, $this->ttl);
+        return (bool) $this->redis->expire($this->prefix.$sessionId, (int) ($this->ttl ?? ini_get('session.gc_maxlifetime')));
     }
 }
diff --git a/Session/Storage/Handler/SessionHandlerFactory.php b/Session/Storage/Handler/SessionHandlerFactory.php
index f4feeac09..a5ebd29eb 100644
--- a/Session/Storage/Handler/SessionHandlerFactory.php
+++ b/Session/Storage/Handler/SessionHandlerFactory.php
@@ -27,7 +27,7 @@ class SessionHandlerFactory
     public static function createHandler($connection): AbstractSessionHandler
     {
         if (!\is_string($connection) && !\is_object($connection)) {
-            throw new \TypeError(sprintf('Argument 1 passed to %s() must be a string or a connection object, %s given.', __METHOD__, \gettype($connection)));
+            throw new \TypeError(sprintf('Argument 1 passed to "%s()" must be a string or a connection object, "%s" given.', __METHOD__, \gettype($connection)));
         }
 
         switch (true) {
@@ -46,17 +46,17 @@ public static function createHandler($connection): AbstractSessionHandler
                 return new PdoSessionHandler($connection);
 
             case !\is_string($connection):
-                throw new \InvalidArgumentException(sprintf('Unsupported Connection: %s.', \get_class($connection)));
+                throw new \InvalidArgumentException(sprintf('Unsupported Connection: "%s".', \get_class($connection)));
             case 0 === strpos($connection, 'file://'):
                 return new StrictSessionHandler(new NativeFileSessionHandler(substr($connection, 7)));
 
-            case 0 === strpos($connection, 'redis://'):
-            case 0 === strpos($connection, 'rediss://'):
-            case 0 === strpos($connection, 'memcached://'):
+            case 0 === strpos($connection, 'redis:'):
+            case 0 === strpos($connection, 'rediss:'):
+            case 0 === strpos($connection, 'memcached:'):
                 if (!class_exists(AbstractAdapter::class)) {
-                    throw new InvalidArgumentException(sprintf('Unsupported DSN "%s". Try running "composer require symfony/cache".', $connection));
+                    throw new \InvalidArgumentException(sprintf('Unsupported DSN "%s". Try running "composer require symfony/cache".', $connection));
                 }
-                $handlerClass = 0 === strpos($connection, 'memcached://') ? MemcachedSessionHandler::class : RedisSessionHandler::class;
+                $handlerClass = 0 === strpos($connection, 'memcached:') ? MemcachedSessionHandler::class : RedisSessionHandler::class;
                 $connection = AbstractAdapter::createConnection($connection, ['lazy' => true]);
 
                 return new $handlerClass($connection);
@@ -80,6 +80,6 @@ public static function createHandler($connection): AbstractSessionHandler
                 return new PdoSessionHandler($connection);
         }
 
-        throw new \InvalidArgumentException(sprintf('Unsupported Connection: %s.', $connection));
+        throw new \InvalidArgumentException(sprintf('Unsupported Connection: "%s".', $connection));
     }
 }
diff --git a/Session/Storage/MockArraySessionStorage.php b/Session/Storage/MockArraySessionStorage.php
index 37b6f145b..db8f85e75 100644
--- a/Session/Storage/MockArraySessionStorage.php
+++ b/Session/Storage/MockArraySessionStorage.php
@@ -148,7 +148,7 @@ public function setName($name)
     public function save()
     {
         if (!$this->started || $this->closed) {
-            throw new \RuntimeException('Trying to save a session that was not started yet or was already closed');
+            throw new \RuntimeException('Trying to save a session that was not started yet or was already closed.');
         }
         // nothing to do since we don't persist the session data
         $this->closed = false;
@@ -186,7 +186,7 @@ public function registerBag(SessionBagInterface $bag)
     public function getBag($name)
     {
         if (!isset($this->bags[$name])) {
-            throw new \InvalidArgumentException(sprintf('The SessionBagInterface %s is not registered.', $name));
+            throw new \InvalidArgumentException(sprintf('The SessionBagInterface "%s" is not registered.', $name));
         }
 
         if (!$this->started) {
diff --git a/Session/Storage/MockFileSessionStorage.php b/Session/Storage/MockFileSessionStorage.php
index 02fe4dad4..c96b3cd9d 100644
--- a/Session/Storage/MockFileSessionStorage.php
+++ b/Session/Storage/MockFileSessionStorage.php
@@ -37,7 +37,7 @@ public function __construct(string $savePath = null, string $name = 'MOCKSESSID'
         }
 
         if (!is_dir($savePath) && !@mkdir($savePath, 0777, true) && !is_dir($savePath)) {
-            throw new \RuntimeException(sprintf('Session Storage was not able to create directory "%s"', $savePath));
+            throw new \RuntimeException(sprintf('Session Storage was not able to create directory "%s".', $savePath));
         }
 
         $this->savePath = $savePath;
@@ -87,7 +87,7 @@ public function regenerate($destroy = false, $lifetime = null)
     public function save()
     {
         if (!$this->started) {
-            throw new \RuntimeException('Trying to save a session that was not started yet or was already closed');
+            throw new \RuntimeException('Trying to save a session that was not started yet or was already closed.');
         }
 
         $data = $this->data;
diff --git a/Session/Storage/NativeSessionStorage.php b/Session/Storage/NativeSessionStorage.php
index 3bc2b2eb4..54933d953 100644
--- a/Session/Storage/NativeSessionStorage.php
+++ b/Session/Storage/NativeSessionStorage.php
@@ -17,6 +17,11 @@
 use Symfony\Component\HttpFoundation\Session\Storage\Proxy\AbstractProxy;
 use Symfony\Component\HttpFoundation\Session\Storage\Proxy\SessionHandlerProxy;
 
+// Help opcache.preload discover always-needed symbols
+class_exists(MetadataBag::class);
+class_exists(StrictSessionHandler::class);
+class_exists(SessionHandlerProxy::class);
+
 /**
  * This provides a base class for session attribute storage.
  *
@@ -139,7 +144,7 @@ public function start()
             return true;
         }
 
-        if (\PHP_SESSION_ACTIVE === session_status()) {
+        if (PHP_SESSION_ACTIVE === session_status()) {
             throw new \RuntimeException('Failed to start the session: already started by PHP.');
         }
 
@@ -149,7 +154,7 @@ public function start()
 
         // ok to try and start the session
         if (!session_start()) {
-            throw new \RuntimeException('Failed to start the session');
+            throw new \RuntimeException('Failed to start the session.');
         }
 
         if (null !== $this->emulateSameSite) {
@@ -202,7 +207,7 @@ public function setName($name)
     public function regenerate($destroy = false, $lifetime = null)
     {
         // Cannot regenerate the session ID for non-active sessions.
-        if (\PHP_SESSION_ACTIVE !== session_status()) {
+        if (PHP_SESSION_ACTIVE !== session_status()) {
             return false;
         }
 
@@ -311,7 +316,7 @@ public function registerBag(SessionBagInterface $bag)
     public function getBag($name)
     {
         if (!isset($this->bags[$name])) {
-            throw new \InvalidArgumentException(sprintf('The SessionBagInterface %s is not registered.', $name));
+            throw new \InvalidArgumentException(sprintf('The SessionBagInterface "%s" is not registered.', $name));
         }
 
         if (!$this->started && $this->saveHandler->isActive()) {
@@ -362,7 +367,7 @@ public function isStarted()
      */
     public function setOptions(array $options)
     {
-        if (headers_sent() || \PHP_SESSION_ACTIVE === session_status()) {
+        if (headers_sent() || PHP_SESSION_ACTIVE === session_status()) {
             return;
         }
 
@@ -401,13 +406,11 @@ public function setOptions(array $options)
      *     ini_set('session.save_path', '/tmp');
      *
      * or pass in a \SessionHandler instance which configures session.save_handler in the
-     * constructor, for a template see NativeFileSessionHandler or use handlers in
-     * composer package drak/native-session
+     * constructor, for a template see NativeFileSessionHandler.
      *
      * @see https://php.net/session-set-save-handler
      * @see https://php.net/sessionhandlerinterface
      * @see https://php.net/sessionhandler
-     * @see https://github.com/zikula/NativeSession
      *
      * @param AbstractProxy|\SessionHandlerInterface|null $saveHandler
      *
@@ -429,7 +432,7 @@ public function setSaveHandler($saveHandler = null)
         }
         $this->saveHandler = $saveHandler;
 
-        if (headers_sent() || \PHP_SESSION_ACTIVE === session_status()) {
+        if (headers_sent() || PHP_SESSION_ACTIVE === session_status()) {
             return;
         }
 
diff --git a/Session/Storage/Proxy/AbstractProxy.php b/Session/Storage/Proxy/AbstractProxy.php
index 0303729e7..b9c2682b3 100644
--- a/Session/Storage/Proxy/AbstractProxy.php
+++ b/Session/Storage/Proxy/AbstractProxy.php
@@ -65,7 +65,7 @@ public function isWrapper()
      */
     public function isActive()
     {
-        return \PHP_SESSION_ACTIVE === session_status();
+        return PHP_SESSION_ACTIVE === session_status();
     }
 
     /**
@@ -88,7 +88,7 @@ public function getId()
     public function setId($id)
     {
         if ($this->isActive()) {
-            throw new \LogicException('Cannot change the ID of an active session');
+            throw new \LogicException('Cannot change the ID of an active session.');
         }
 
         session_id($id);
@@ -114,7 +114,7 @@ public function getName()
     public function setName($name)
     {
         if ($this->isActive()) {
-            throw new \LogicException('Cannot change the name of an active session');
+            throw new \LogicException('Cannot change the name of an active session.');
         }
 
         session_name($name);
diff --git a/Tests/RequestTest.php b/Tests/RequestTest.php
index 1d0164725..7c53ec2d5 100644
--- a/Tests/RequestTest.php
+++ b/Tests/RequestTest.php
@@ -408,12 +408,10 @@ public function testGetPreferredFormat()
 
         $request->setRequestFormat('atom');
         $request->headers->set('Accept', 'application/ld+json');
-        $request->headers->set('Content-Type', 'application/merge-patch+json');
         $this->assertSame('atom', $request->getPreferredFormat());
 
         $request = new Request();
         $request->headers->set('Accept', 'application/xml');
-        $request->headers->set('Content-Type', 'application/json');
         $this->assertSame('xml', $request->getPreferredFormat());
 
         $request = new Request();
diff --git a/Tests/ResponseFunctionalTest.php b/Tests/ResponseFunctionalTest.php
index 21a66bbf8..849a1395d 100644
--- a/Tests/ResponseFunctionalTest.php
+++ b/Tests/ResponseFunctionalTest.php
@@ -13,9 +13,6 @@
 
 use PHPUnit\Framework\TestCase;
 
-/**
- * @requires PHP 7.0
- */
 class ResponseFunctionalTest extends TestCase
 {
     private static $server;
diff --git a/Tests/ResponseHeaderBagTest.php b/Tests/ResponseHeaderBagTest.php
index 1a3817333..18b3c36a6 100644
--- a/Tests/ResponseHeaderBagTest.php
+++ b/Tests/ResponseHeaderBagTest.php
@@ -51,9 +51,9 @@ public function testCacheControlHeader()
         $this->assertTrue($bag->hasCacheControlDirective('public'));
 
         $bag = new ResponseHeaderBag(['ETag' => 'abcde']);
-        $this->assertEquals('private, must-revalidate', $bag->get('Cache-Control'));
+        $this->assertEquals('no-cache, private', $bag->get('Cache-Control'));
         $this->assertTrue($bag->hasCacheControlDirective('private'));
-        $this->assertTrue($bag->hasCacheControlDirective('must-revalidate'));
+        $this->assertTrue($bag->hasCacheControlDirective('no-cache'));
         $this->assertFalse($bag->hasCacheControlDirective('max-age'));
 
         $bag = new ResponseHeaderBag(['Expires' => 'Wed, 16 Feb 2011 14:17:43 GMT']);
@@ -128,6 +128,14 @@ public function testClearCookieSecureNotHttpOnly()
         $this->assertSetCookieHeader('foo=deleted; expires='.gmdate('D, d-M-Y H:i:s T', time() - 31536001).'; Max-Age=0; path=/; secure', $bag);
     }
 
+    public function testClearCookieSamesite()
+    {
+        $bag = new ResponseHeaderBag([]);
+
+        $bag->clearCookie('foo', '/', null, true, false, 'none');
+        $this->assertSetCookieHeader('foo=deleted; expires='.gmdate('D, d-M-Y H:i:s T', time() - 31536001).'; Max-Age=0; path=/; secure; samesite=none', $bag);
+    }
+
     public function testReplace()
     {
         $bag = new ResponseHeaderBag([]);
diff --git a/Tests/ResponseTest.php b/Tests/ResponseTest.php
index a2a5574f2..c766f88a2 100644
--- a/Tests/ResponseTest.php
+++ b/Tests/ResponseTest.php
@@ -461,18 +461,10 @@ public function testSetVary()
 
     public function testDefaultContentType()
     {
-        $headerMock = $this->getMockBuilder('Symfony\Component\HttpFoundation\ResponseHeaderBag')->setMethods(['set'])->getMock();
-        $headerMock->expects($this->at(0))
-            ->method('set')
-            ->with('Content-Type', 'text/html');
-        $headerMock->expects($this->at(1))
-            ->method('set')
-            ->with('Content-Type', 'text/html; charset=UTF-8');
-
         $response = new Response('foo');
-        $response->headers = $headerMock;
-
         $response->prepare(new Request());
+
+        $this->assertSame('text/html; charset=UTF-8', $response->headers->get('Content-Type'));
     }
 
     public function testContentTypeCharset()
@@ -505,12 +497,25 @@ public function testPrepareDoesNothingIfRequestFormatIsNotDefined()
         $this->assertEquals('text/html; charset=UTF-8', $response->headers->get('content-type'));
     }
 
+    /**
+     * Same URL cannot produce different Content-Type based on the value of the Accept header,
+     * unless explicitly stated in the response object.
+     */
+    public function testPrepareDoesNotSetContentTypeBasedOnRequestAcceptHeader()
+    {
+        $response = new Response('foo');
+        $request = Request::create('/');
+        $request->headers->set('Accept', 'application/json');
+        $response->prepare($request);
+
+        $this->assertSame('text/html; charset=UTF-8', $response->headers->get('content-type'));
+    }
+
     public function testPrepareSetContentType()
     {
         $response = new Response('foo');
         $request = Request::create('/');
         $request->setRequestFormat('json');
-        $request->headers->remove('accept');
 
         $response->prepare($request);
 
diff --git a/Tests/Session/Storage/PhpBridgeSessionStorageTest.php b/Tests/Session/Storage/PhpBridgeSessionStorageTest.php
index 206ff4877..b4fad7688 100644
--- a/Tests/Session/Storage/PhpBridgeSessionStorageTest.php
+++ b/Tests/Session/Storage/PhpBridgeSessionStorageTest.php
@@ -61,12 +61,12 @@ public function testPhpSession()
     {
         $storage = $this->getStorage();
 
-        $this->assertNotSame(\PHP_SESSION_ACTIVE, session_status());
+        $this->assertNotSame(PHP_SESSION_ACTIVE, session_status());
         $this->assertFalse($storage->isStarted());
 
         session_start();
         $this->assertTrue(isset($_SESSION));
-        $this->assertSame(\PHP_SESSION_ACTIVE, session_status());
+        $this->assertSame(PHP_SESSION_ACTIVE, session_status());
         // PHP session might have started, but the storage driver has not, so false is correct here
         $this->assertFalse($storage->isStarted());