diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 77458a1..00be5b0 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -2507,7 +2507,7 @@ function typed_data() {
 function drupal_valid_test_ua() {
   global $drupal_hash_salt;
   // No reason to reset this.
-  static $test_prefix;
+//  static $test_prefix;
 
   if (isset($test_prefix)) {
     return $test_prefix;
diff --git a/core/lib/Drupal/Core/Cache/ApcBackend.php b/core/lib/Drupal/Core/Cache/ApcBackend.php
new file mode 100644
index 0000000..3bb771e
--- /dev/null
+++ b/core/lib/Drupal/Core/Cache/ApcBackend.php
@@ -0,0 +1,380 @@
+<?php
+
+/**
+ * @file
+ * Contains Drupal\Core\Cache\ApcBackend.
+ */
+
+namespace Drupal\Core\Cache;
+
+/**
+ * Defines the APC cache implementation.
+ *
+ * Stores cache items in the Alternative PHP Cache (APC). Each cache bin
+ * corresponds to a prefix of the APC variable with the same name.
+ */
+class ApcBackend implements CacheBackendInterface {
+
+  /**
+   * @var string
+   */
+  protected $bin;
+
+  /**
+   * @var string
+   */
+  protected $prefix;
+
+  /**
+   * A static cache of all tags checked during the request.
+   */
+  protected static $tagCache = array();
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::__construct().
+   */
+  public function __construct($bin) {
+    $this->bin = $bin;
+
+    // First we determine the prefix from a setting.
+    $prefix = self::getPrefixSettingForBin($this->bin);
+
+    // If we do not have a configured prefix we use the HTTP_HOST.
+    if (empty($prefix) && isset($_SERVER['HTTP_HOST'])) {
+      // Provide a fallback for multisite. This is on purpose not inside the
+      // getPrefixForBin() function in order to decouple the unified prefix
+      // variable logic and custom module related security logic, that is not
+      // necessary for all backends.
+      $prefix = $_SERVER['HTTP_HOST'] . '::';
+    }
+    else {
+      $prefix = $prefix . '::';
+    }
+
+    // When we are in testing mode we add the test prefix.
+    if ($test_prefix = drupal_valid_test_ua()) {
+      debug('prefix');
+      $prefix = $test_prefix . '::' . $prefix;
+    }
+    elseif (isset($GLOBALS['drupal_test_info'])) {
+      debug('global');
+      $prefix = $GLOBALS['drupal_test_info']['test_run_id'] . '::' . $prefix;
+    }
+
+    $this->prefix = $prefix;
+  }
+
+  /**
+   * Get prefix for bin using the configuration.
+   *
+   * @param string $bin
+   *
+   * @return string
+   *   Can be an empty string, if no prefix set.
+   */
+  protected static function getPrefixSettingForBin($bin) {
+    $prefixes = variable_get('cache_prefix', '');
+
+    if (is_string($prefixes)) {
+      // Variable can be a string, which then considered as a default behavior.
+      return $prefixes;
+    }
+
+    if (isset($prefixes[$bin])) {
+      if (FALSE !== $prefixes[$bin]) {
+        // If entry is set and not FALSE, an explicit prefix is set for the bin.
+        return $prefixes[$bin];
+      }
+      else {
+        // If we have an explicit false, it means no prefix whatever is the
+        // default configuration.
+        return '';
+      }
+    }
+    else {
+      // Key is not set, we can safely rely on default behavior.
+      if (isset($prefixes['default']) && FALSE !== $prefixes['default']) {
+        return $prefixes['default'];
+      }
+      else {
+        // When default is not set or an explicit FALSE, this means no prefix.
+        return '';
+      }
+    }
+  }
+
+  /**
+   * Function which retrieves the safe key for the cache bin.
+   *
+   * @return string
+   *   The safe APC key.
+   */
+  protected function binKey() {
+    return $this->prefix . $this->bin . '::';
+  }
+
+  /**
+   * Function which retrieves the safe key for the cache cid.
+   *
+   * @param string $cid
+   *   The cache id.
+   * @return string
+   *   The safe APC key.
+   */
+  protected function key($cid) {
+    return $this->binKey() . $cid;
+  }
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::get().
+   */
+  public function get($cid) {
+    $cache = apc_fetch($this->key($cid));
+    return $this->prepareItem($cache);
+  }
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::getMultiple().
+   */
+  public function getMultiple(&$cids) {
+    // Translate the requested cache item keys to APC keys.
+    $map = array();
+    foreach ($cids as $cid) {
+      $map[$this->key($cid)] = $cid;
+    }
+
+    $result = apc_fetch(array_keys($map));
+    $cache = array();
+    foreach ($result as $key => $item) {
+      $item = $this->prepareItem($item);
+      if ($item) {
+        $cache[$map[$key]] = $item;
+      }
+    }
+    unset($result);
+
+    $cids = array_diff($cids, array_keys($cache));
+    return $cache;
+  }
+
+  /**
+   * Returns all cached items, optionally limited by a cache ID prefix.
+   *
+   * APC is a memory cache, shared across all server processes. To prevent cache
+   * item clashes with other applications/installations, every cache item is
+   * prefixed with a unique string for this application. Therefore, functions
+   * like apc_clear_cache() cannot be used, and instead, a list of all cache
+   * items belonging to this application need to be retrieved through this
+   * method instead.
+   *
+   * @param string $prefix
+   *   (optional) A cache ID prefix to limit the result to.
+   *
+   * @return APCIterator
+   *   An APCIterator containing matched items.
+   */
+  public function getAll($prefix = '') {
+    return new \APCIterator('user', '/^' . preg_quote($this->binKey() . $prefix, '/') . '/', APC_ITER_KEY);
+  }
+
+  /**
+   * Prepares a cached item.
+   *
+   * Checks that items are either permanent or did not expire.
+   *
+   * @param stdClass $cache
+   *   An item loaded from cache_get() or cache_get_multiple().
+   *
+   * @return mixed
+   *   The item with data unserialized as appropriate or FALSE if there is no
+   *   valid item to load.
+   */
+  protected function prepareItem($cache) {
+    if (!isset($cache->data)) {
+      return FALSE;
+    }
+
+    // The cache data is invalid if any of its tags have been cleared since.
+    if ($cache->tags) {
+      $cache->tags = explode(' ', $cache->tags);
+      if (!$this->validTags($cache->checksum, $cache->tags)) {
+        return FALSE;
+      }
+    }
+
+    return $cache;
+  }
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::set().
+   */
+  public function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array()) {
+    $cache = new \stdClass();
+    $cache->cid = $cid;
+    $cache->created = REQUEST_TIME;
+    $cache->expire = $expire;
+    $cache->tags = implode(' ', $this->flattenTags($tags));
+    $cache->checksum = $this->checksumTags($tags);
+
+    // APC serializes/unserializes any structure itself.
+    $cache->serialized = 0;
+    $cache->data = $data;
+
+    // apc_store()'s $ttl argument can be omitted but also set to 0 (zero),
+    // which happens to be identical to CACHE_PERMANENT.
+    apc_store($this->key($cid), $cache, $expire);
+  }
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::delete().
+   */
+  public function delete($cid) {
+    apc_delete($this->key($cid));
+  }
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::deleteMultiple().
+   */
+  public function deleteMultiple(array $cids) {
+    apc_delete(array_map(array($this, 'key'), $cids));
+  }
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::flush().
+   */
+  public function flush() {
+    $iterator = $this->getAll();
+//    debug($iterator);
+    foreach ($iterator as $key => $data) {
+//      debug($key);
+      apc_delete($key);
+    }
+  }
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::expire().
+   */
+  public function expire() {
+    // Any call to apc_fetch() causes APC to expunge expired items.
+    apc_fetch('');
+  }
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::garbageCollection().
+   */
+  public function garbageCollection() {
+    $this->expire();
+  }
+
+  /**
+   * Compares two checksums of tags. Used to determine whether to serve a cached
+   * item or treat it as invalidated.
+   *
+   * @param integer $checksum
+   *   The initial checksum to compare against.
+   * @param array $tags
+   *   An array of tags to calculate a checksum for.
+   *
+   * @return boolean
+   *   TRUE if the checksums match, FALSE otherwise.
+   */
+  protected function validTags($checksum, array $tags) {
+    return $checksum == $this->checksumTags($tags);
+  }
+
+  /**
+   * Flattens a tags array into a numeric array suitable for string storage.
+   *
+   * @param array $tags
+   *   Associative array of tags to flatten.
+   *
+   * @return array
+   *   Numeric array of flattened tag identifiers.
+   */
+  protected function flattenTags(array $tags) {
+    if (isset($tags[0])) {
+      return $tags;
+    }
+
+    $flat_tags = array();
+    foreach ($tags as $namespace => $values) {
+      if (is_array($values)) {
+        foreach ($values as $value) {
+          $flat_tags[] = "$namespace:$value";
+        }
+      }
+      else {
+        $flat_tags[] = "$namespace:$values";
+      }
+    }
+    return $flat_tags;
+  }
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::invalidateTags().
+   */
+  public function invalidateTags(array $tags) {
+    foreach ($this->flattenTags($tags) as $tag) {
+      unset(self::$tagCache[$tag]);
+      apc_inc($this->prefix . 'tags::' . $tag, 1, $success);
+      if (!$success) {
+        apc_store($this->prefix . 'tags::' . $tag, 1);
+      }
+    }
+  }
+
+  /**
+   * Returns the sum total of validations for a given set of tags.
+   *
+   * @param array $tags
+   *   Associative array of tags.
+   *
+   * @return integer
+   *   Sum of all invalidations.
+   */
+  protected function checksumTags($tags) {
+    $checksum = 0;
+    $query_tags = array();
+
+    foreach ($this->flattenTags($tags) as $tag) {
+      if (isset(self::$tagCache[$tag])) {
+        $checksum += self::$tagCache[$tag];
+      }
+      else {
+        $query_tags[] = $this->prefix . 'tags::' . $tag;
+      }
+   }
+    if ($query_tags) {
+      $result = apc_fetch($query_tags);
+      self::$tagCache = array_merge(self::$tagCache, $result);
+      $checksum += array_sum($result);
+    }
+    return $checksum;
+  }
+
+  /**
+   * Implements Drupal\Core\Cache\CacheBackendInterface::isEmpty().
+   */
+  public function isEmpty() {
+    return $this->getAll()->getTotalCount() === 0;
+  }
+}
+
+// PHP CLI uses a different php.ini in which APC may not be enabled.
+// Create stub functions to prevent fatal errors.
+if (drupal_is_cli() && !function_exists('apc_fetch')) {
+  function apc_fetch($key, &$success) {
+    $success = TRUE;
+    return FALSE;
+  }
+  function apc_store($key, $var) {
+    return FALSE;
+  }
+  function apc_delete($key) {
+    return FALSE;
+  }
+  function apc_inc($key, $step, &$success) {
+    $success = TRUE;
+    return FALSE;
+  }
+}
diff --git a/core/lib/Drupal/Core/Cache/CacheBackendInterface.php b/core/lib/Drupal/Core/Cache/CacheBackendInterface.php
index 02c9dd2..9c4f8fc 100644
--- a/core/lib/Drupal/Core/Cache/CacheBackendInterface.php
+++ b/core/lib/Drupal/Core/Cache/CacheBackendInterface.php
@@ -38,8 +38,8 @@
  * To access your custom cache bin, specify the name of the bin when storing
  * or retrieving cached data:
  * @code
- *  cache_set($cid, $data, 'custom_bin', $expire);
- *  cache_get($cid, 'custom_bin');
+ *  cache('custom_bin')->set($cid, $data, $expire);
+ *  cache('custom_bin')->get($cid);
  * @endcode
  *
  * @see cache()
@@ -60,7 +60,7 @@
    * @param $bin
    *   The cache bin for which the object is created.
    */
-  function __construct($bin);
+  public function __construct($bin);
 
   /**
    * Returns data from the persistent cache.
@@ -74,7 +74,7 @@ function __construct($bin);
    * @return
    *   The cache or FALSE on failure.
    */
-  function get($cid);
+  public function get($cid);
 
   /**
    * Returns data from the persistent cache when given an array of cache IDs.
@@ -87,7 +87,7 @@ function get($cid);
    * @return
    *   An array of the items successfully returned from cache indexed by cid.
    */
-  function getMultiple(&$cids);
+  public function getMultiple(&$cids);
 
   /**
    * Stores data in the persistent cache.
@@ -111,7 +111,7 @@ function getMultiple(&$cids);
    *   a node, both the node ID and the author's user ID might be passed in as
    *   tags. For example array('node' => array(123), 'user' => array(92)).
    */
-  function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array());
+  public function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array());
 
   /**
    * Deletes an item from the cache.
@@ -119,7 +119,7 @@ function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, arra
    * @param $cid
    *    The cache ID to delete.
    */
-  function delete($cid);
+  public function delete($cid);
 
   /**
    * Deletes multiple items from the cache.
@@ -127,17 +127,17 @@ function delete($cid);
    * @param $cids
    *   An array of $cids to delete.
    */
-  function deleteMultiple(Array $cids);
+  public function deleteMultiple(Array $cids);
 
   /**
    * Flushes all cache items in a bin.
    */
-  function flush();
+  public function flush();
 
   /**
    * Expires temporary items from the cache.
    */
-  function expire();
+  public function expire();
 
   /**
    * Invalidates each tag in the $tags array.
@@ -148,12 +148,12 @@ function expire();
    *
    * @see CacheBackendInterface::set()
    */
-  function invalidateTags(array $tags);
+  public function invalidateTags(array $tags);
 
   /**
    * Performs garbage collection on a cache bin.
    */
-  function garbageCollection();
+  public function garbageCollection();
 
   /**
    * Checks if a cache bin is empty.
@@ -164,5 +164,5 @@ function garbageCollection();
    * @return
    *   TRUE if the cache bin specified is empty.
    */
-  function isEmpty();
+  public function isEmpty();
 }
diff --git a/core/lib/Drupal/Core/Cache/DatabaseBackend.php b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
index e62f1da..9ab6b11 100644
--- a/core/lib/Drupal/Core/Cache/DatabaseBackend.php
+++ b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
@@ -31,7 +31,7 @@ class DatabaseBackend implements CacheBackendInterface {
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::__construct().
    */
-  function __construct($bin) {
+  public function __construct($bin) {
     // All cache tables should be prefixed with 'cache_', except for the
     // default 'cache' bin.
     if ($bin != 'cache') {
@@ -43,7 +43,7 @@ function __construct($bin) {
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::get().
    */
-  function get($cid) {
+  public function get($cid) {
     $cids = array($cid);
     $cache = $this->getMultiple($cids);
     return reset($cache);
@@ -52,7 +52,7 @@ function get($cid) {
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::getMultiple().
    */
-  function getMultiple(&$cids) {
+  public function getMultiple(&$cids) {
     try {
       // When serving cached pages, the overhead of using ::select() was found
       // to add around 30% overhead to the request. Since $this->bin is a
@@ -93,8 +93,6 @@ function getMultiple(&$cids) {
    *   valid item to load.
    */
   protected function prepareItem($cache) {
-    global $user;
-
     if (!isset($cache->data)) {
       return FALSE;
     }
@@ -118,7 +116,7 @@ protected function prepareItem($cache) {
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::set().
    */
-  function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array()) {
+  public function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array()) {
     $fields = array(
       'serialized' => 0,
       'created' => REQUEST_TIME,
@@ -149,7 +147,7 @@ function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, arra
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::delete().
    */
-  function delete($cid) {
+  public function delete($cid) {
     Database::getConnection()->delete($this->bin)
       ->condition('cid', $cid)
       ->execute();
@@ -158,7 +156,7 @@ function delete($cid) {
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::deleteMultiple().
    */
-  function deleteMultiple(array $cids) {
+  public function deleteMultiple(array $cids) {
     // Delete in chunks when a large array is passed.
     do {
       Database::getConnection()->delete($this->bin)
@@ -171,14 +169,14 @@ function deleteMultiple(array $cids) {
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::flush().
    */
-  function flush() {
+  public function flush() {
     Database::getConnection()->truncate($this->bin)->execute();
   }
 
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::expire().
    */
-  function expire() {
+  public function expire() {
     Database::getConnection()->delete($this->bin)
       ->condition('expire', CacheBackendInterface::CACHE_PERMANENT, '<>')
       ->condition('expire', REQUEST_TIME, '<')
@@ -188,7 +186,7 @@ function expire() {
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::garbageCollection().
    */
-  function garbageCollection() {
+  public function garbageCollection() {
     $this->expire();
   }
 
@@ -196,9 +194,9 @@ function garbageCollection() {
    * Compares two checksums of tags. Used to determine whether to serve a cached
    * item or treat it as invalidated.
    *
-   * @param integer @checksum
+   * @param integer $checksum
    *   The initial checksum to compare against.
-   * @param array @tags
+   * @param array $tags
    *   An array of tags to calculate a checksum for.
    *
    * @return boolean
@@ -214,7 +212,7 @@ protected function validTags($checksum, array $tags) {
    * @param array $tags
    *   Associative array of tags to flatten.
    *
-   * @return
+   * @return array
    *   Numeric array of flattened tag identifiers.
    */
   protected function flattenTags(array $tags) {
@@ -288,7 +286,7 @@ protected function checksumTags($tags) {
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::isEmpty().
    */
-  function isEmpty() {
+  public function isEmpty() {
     $this->garbageCollection();
     $query = Database::getConnection()->select($this->bin);
     $query->addExpression('1');
diff --git a/core/lib/Drupal/Core/Cache/InstallBackend.php b/core/lib/Drupal/Core/Cache/InstallBackend.php
index 73aae39..e23294a 100644
--- a/core/lib/Drupal/Core/Cache/InstallBackend.php
+++ b/core/lib/Drupal/Core/Cache/InstallBackend.php
@@ -36,26 +36,26 @@ class InstallBackend extends DatabaseBackend {
   /**
    * Overrides Drupal\Core\Cache\DatabaseBackend::get().
    */
-  function get($cid) {
+  public function get($cid) {
     return FALSE;
   }
 
   /**
    * Overrides Drupal\Core\Cache\DatabaseBackend::getMultiple().
    */
-  function getMultiple(&$cids) {
+  public function getMultiple(&$cids) {
     return array();
   }
 
   /**
    * Overrides Drupal\Core\Cache\DatabaseBackend::set().
    */
-  function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array()) {}
+  public function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array()) {}
 
   /**
    * Overrides Drupal\Core\Cache\DatabaseBackend::delete().
    */
-  function delete($cid) {
+  public function delete($cid) {
     try {
       if (class_exists('Drupal\Core\Database\Database')) {
         parent::delete($cid);
@@ -67,7 +67,7 @@ function delete($cid) {
   /**
    * Overrides Drupal\Core\Cache\DatabaseBackend::deleteMultiple().
    */
-  function deleteMultiple(array $cids) {
+  public function deleteMultiple(array $cids) {
     try {
       if (class_exists('Drupal\Core\Database\Database')) {
         parent::deleteMultiple($cids);
@@ -79,7 +79,7 @@ function deleteMultiple(array $cids) {
   /**
    * Overrides Drupal\Core\Cache\DatabaseBackend::invalidateTags().
    */
-  function invalidateTags(array $tags) {
+  public function invalidateTags(array $tags) {
     try {
       if (class_exists('Drupal\Core\Database\Database')) {
         parent::invalidateTags($tags);
@@ -91,7 +91,7 @@ function invalidateTags(array $tags) {
   /**
    * Overrides Drupal\Core\Cache\DatabaseBackend::flush().
    */
-  function flush() {
+  public function flush() {
     try {
       if (class_exists('Drupal\Core\Database\Database')) {
         parent::flush();
@@ -103,7 +103,7 @@ function flush() {
   /**
    * Overrides Drupal\Core\Cache\DatabaseBackend::expire().
    */
-  function expire() {
+  public function expire() {
     try {
       if (class_exists('Drupal\Core\Database\Database')) {
         parent::expire();
@@ -115,7 +115,7 @@ function expire() {
   /**
    * Overrides Drupal\Core\Cache\DatabaseBackend::garbageCollection().
    */
-  function garbageCollection() {
+  public function garbageCollection() {
     try {
       if (class_exists('Drupal\Core\Database\Database')) {
         parent::garbageCollection();
@@ -127,7 +127,7 @@ function garbageCollection() {
   /**
    * Overrides Drupal\Core\Cache\DatabaseBackend::isEmpty().
    */
-  function isEmpty() {
+  public function isEmpty() {
     try {
       if (class_exists('Drupal\Core\Database\Database')) {
         return parent::isEmpty();
diff --git a/core/lib/Drupal/Core/Cache/MemoryBackend.php b/core/lib/Drupal/Core/Cache/MemoryBackend.php
index ce74c2f..b17e06b 100644
--- a/core/lib/Drupal/Core/Cache/MemoryBackend.php
+++ b/core/lib/Drupal/Core/Cache/MemoryBackend.php
@@ -105,13 +105,13 @@ public function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANEN
     );
   }
 
-  /*
+  /**
    * Calculates a checksum so data can be invalidated using tags.
    */
-  function checksum($tags) {
-    $checksum = "";
+  public function checksum($tags) {
+    $checksum = '';
 
-    foreach($tags as $tag) {
+    foreach ($tags as $tag) {
       // Has the tag already been invalidated.
       if (isset($this->invalidatedTags[$tag])) {
         $checksum = $checksum . $tag . ':' . $this->invalidatedTags[$tag];
@@ -201,8 +201,7 @@ protected function flattenTags(array $tags) {
    * Implements Drupal\Core\Cache\CacheBackendInterface::invalidateTags().
    */
   public function invalidateTags(array $tags) {
-    $flat_tags = $this->flattenTags($tags);
-    foreach($flat_tags as $tag) {
+    foreach ($this->flattenTags($tags) as $tag) {
       if (isset($this->invalidatedTags[$tag])) {
         $this->invalidatedTags[$tag] = $this->invalidatedTags[$tag] + 1;
       }
diff --git a/core/lib/Drupal/Core/Cache/NullBackend.php b/core/lib/Drupal/Core/Cache/NullBackend.php
index 862a3bd..ee8b1d3 100644
--- a/core/lib/Drupal/Core/Cache/NullBackend.php
+++ b/core/lib/Drupal/Core/Cache/NullBackend.php
@@ -23,51 +23,51 @@ class NullBackend implements CacheBackendInterface {
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::__construct().
    */
-  function __construct($bin) {}
+  public function __construct($bin) {}
 
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::get().
    */
-  function get($cid) {
+  public function get($cid) {
     return FALSE;
   }
 
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::getMultiple().
    */
-  function getMultiple(&$cids) {
+  public function getMultiple(&$cids) {
     return array();
   }
 
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::set().
    */
-  function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array()) {}
+  public function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array()) {}
 
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::delete().
    */
-  function delete($cid) {}
+  public function delete($cid) {}
 
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::deleteMultiple().
    */
-  function deleteMultiple(array $cids) {}
+  public function deleteMultiple(array $cids) {}
 
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::flush().
    */
-  function flush() {}
+  public function flush() {}
 
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::expire().
    */
-  function expire() {}
+  public function expire() {}
 
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::garbageCollection().
    */
-  function garbageCollection() {}
+  public function garbageCollection() {}
 
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::invalidateTags().
@@ -77,7 +77,7 @@ public function invalidateTags(array $tags) {}
   /**
    * Implements Drupal\Core\Cache\CacheBackendInterface::isEmpty().
    */
-  function isEmpty() {
+  public function isEmpty() {
     return TRUE;
   }
 }
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index 7320a74..a2548f6 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -829,6 +829,12 @@ protected function tearDown() {
       $this->pass($message, t('E-mail'));
     }
 
+    // Reset all static variables.
+    // Static variables may contain objects with magic destructors; resetting
+    // all statics causes them to be invoked and they might write data based on
+    // the environment information they were given within the test.
+    drupal_static_reset();
+
     // Delete temporary files directory.
     file_unmanaged_delete_recursive($this->originalFileDirectory . '/simpletest/' . substr($this->databasePrefix, 10), array($this, 'filePreDeleteCallback'));
 
@@ -844,9 +850,6 @@ protected function tearDown() {
     $GLOBALS['theme_key'] = $this->originalThemeKey;
     $GLOBALS['theme'] = $this->originalTheme;
 
-    // Reset all static variables.
-    drupal_static_reset();
-
     // Reset module list and module load status.
     module_list_reset();
     module_load_all(FALSE, TRUE);
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
index 98ed71d..7d3251d 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
@@ -67,7 +67,13 @@ protected function setUp() {
     }
 
     // Set user agent to be consistent with WebTestBase.
-    $_SERVER['HTTP_USER_AGENT'] = $this->databasePrefix;
+    // @see WebTestBase::curlInitialize()
+//    $this->originalUserAgent = $_SERVER['HTTP_USER_AGENT'];
+    if (preg_match('/simpletest\d+/', $this->databasePrefix, $matches)) {
+//      debug($this->databasePrefix);
+      $_SERVER['HTTP_USER_AGENT'] = drupal_generate_test_ua($matches[0]);
+//      debug($_SERVER['HTTP_USER_AGENT']);
+    }
 
     $this->setup = TRUE;
   }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Cache/ApcBackendUnitTest.php b/core/modules/system/lib/Drupal/system/Tests/Cache/ApcBackendUnitTest.php
new file mode 100644
index 0000000..1296886
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Tests/Cache/ApcBackendUnitTest.php
@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * @file
+ * Contains Drupal\system\Tests\Cache\ApcBackendUnitTest.
+ */
+
+namespace Drupal\system\Tests\Cache;
+
+use Drupal\Core\Cache\ApcBackend;
+
+/**
+ * Tests the APC cache backend.
+ *
+ * @todo APC still contains the following variables after each tearDown() of a
+ *   test method:
+ *   - cache::theme_registry:build:modules
+ *   - cache::theme_registry:stark
+ *   - tags::*
+ *   The first two are caused by CacheArray::__destruct() being invoked *after*
+ *   TestBase::tearDown() reset static caches.
+ *   Cache tag invalidation counters, however, are not flushed by design. It
+ *   might be possible to flush them in a tearDown() override method in this
+ *   class though.
+ */
+class ApcBackendUnitTest extends GenericCacheBackendUnitTestBase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'APC cache backend',
+      'description' => 'Tests the APC cache backend.',
+      'group' => 'Cache',
+    );
+  }
+
+  protected function createCacheBackend($bin) {
+    return new ApcBackend($bin);
+  }
+}
diff --git a/core/modules/system/lib/Drupal/system/Tests/Cache/MemoryBackendUnitTest.php b/core/modules/system/lib/Drupal/system/Tests/Cache/MemoryBackendUnitTest.php
index 843eb23..499ddaf 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Cache/MemoryBackendUnitTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Cache/MemoryBackendUnitTest.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Definition of Drupal\system\Tests\Cache\ArrayBackendUnitTest.
+ * Definition of Drupal\system\Tests\Cache\MemoryBackendUnitTest.
  */
 
 namespace Drupal\system\Tests\Cache;