diff --git a/CRM/Utils/Cache/APCcache.php b/CRM/Utils/Cache/APCcache.php
index e696aa19c3d2..a8b4e5cd6949 100644
--- a/CRM/Utils/Cache/APCcache.php
+++ b/CRM/Utils/Cache/APCcache.php
@@ -31,6 +31,10 @@
  * @copyright CiviCRM LLC (c) 2004-2018
  */
 class CRM_Utils_Cache_APCcache implements CRM_Utils_Cache_Interface {
+
+  use CRM_Utils_Cache_NaiveMultipleTrait; // TODO Consider native implementation.
+  use CRM_Utils_Cache_NaiveHasTrait; // TODO Native implementation
+
   const DEFAULT_TIMEOUT = 3600;
   const DEFAULT_PREFIX = '';
 
@@ -72,10 +76,14 @@ public function __construct(&$config) {
   /**
    * @param $key
    * @param $value
+   * @param null|int|\DateInterval $ttl
    *
    * @return bool
    */
-  public function set($key, &$value) {
+  public function set($key, $value, $ttl = NULL) {
+    if ($ttl !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::set() should support non-NULL TTL");
+    }
     if (!apc_store($this->_prefix . $key, $value, $this->_timeout)) {
       return FALSE;
     }
@@ -84,10 +92,14 @@ public function set($key, &$value) {
 
   /**
    * @param $key
+   * @param mixed $default
    *
    * @return mixed
    */
-  public function get($key) {
+  public function get($key, $default = NULL) {
+    if ($default !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::get() only supports NULL default");
+    }
     return apc_fetch($this->_prefix . $key);
   }
 
@@ -113,6 +125,11 @@ public function flush() {
         apc_delete($this->_prefix . $name);
       }
     }
+    return TRUE;
+  }
+
+  public function clear() {
+    return $this->flush();
   }
 
 }
diff --git a/CRM/Utils/Cache/ArrayCache.php b/CRM/Utils/Cache/ArrayCache.php
index b2272fa1eb6c..c74e1900f1bc 100644
--- a/CRM/Utils/Cache/ArrayCache.php
+++ b/CRM/Utils/Cache/ArrayCache.php
@@ -36,6 +36,9 @@
  */
 class CRM_Utils_Cache_Arraycache implements CRM_Utils_Cache_Interface {
 
+  use CRM_Utils_Cache_NaiveMultipleTrait;
+  use CRM_Utils_Cache_NaiveHasTrait; // TODO Native implementation
+
   /**
    * The cache storage container, an in memory array by default
    */
@@ -56,30 +59,44 @@ public function __construct($config) {
   /**
    * @param string $key
    * @param mixed $value
+   * @param null|int|\DateInterval $ttl
+   * @return bool
    */
-  public function set($key, &$value) {
+  public function set($key, $value, $ttl = NULL) {
+    if ($ttl !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::set() should support non-NULL TTL");
+    }
     $this->_cache[$key] = $value;
+    return TRUE;
   }
 
   /**
    * @param string $key
+   * @param mixed $default
    *
    * @return mixed
    */
-  public function get($key) {
-    return CRM_Utils_Array::value($key, $this->_cache);
+  public function get($key, $default = NULL) {
+    return CRM_Utils_Array::value($key, $this->_cache, $default);
   }
 
   /**
    * @param string $key
+   * @return bool
    */
   public function delete($key) {
     unset($this->_cache[$key]);
+    return TRUE;
   }
 
   public function flush() {
     unset($this->_cache);
     $this->_cache = array();
+    return TRUE;
+  }
+
+  public function clear() {
+    return $this->flush();
   }
 
 }
diff --git a/CRM/Utils/Cache/Interface.php b/CRM/Utils/Cache/Interface.php
index f11327cb59c7..9effdd9d3aa6 100644
--- a/CRM/Utils/Cache/Interface.php
+++ b/CRM/Utils/Cache/Interface.php
@@ -30,60 +30,78 @@
  * @package CRM
  * @copyright CiviCRM LLC (c) 2004-2018
  *
- * CRM_Utils_Cache_Interface
+ * CRM_Utils_Cache_Interface is a long-standing interface used within CiviCRM
+ * for interacting with a cache service. In style and substance, it is extremely
+ * similar to PHP-FIG's SimpleCache interface (PSR-16). Consequently, beginning
+ * with CiviCRM v5.4, this extends \Psr\SimpleCache\CacheInterface.
  *
- * PHP-FIG has been developing a draft standard for caching,
- * PSR-6. The standard has not been ratified yet. When
- * making changes to this interface, please take care to
- * avoid *conflicst* with PSR-6's CacheItemPoolInterface. At
- * time of writing, they do not conflict. Avoiding conflicts
- * will enable more transition paths where Civi
- * simultaneously supports both interfaces in the same
- * implementation.
- *
- * For example, the current interface defines:
- *
- *   function get($key) => mixed $value
- *
- * and PSR-6 defines:
- *
- *   function getItem($key) => ItemInterface $item
- *
- * These are different styles (e.g. "weak item" vs "strong item"),
- * but the two methods do not *conflict*. They can coexist,
- * and you can trivially write adapters between the two.
- *
- * @see https://github.com/php-fig/fig-standards/blob/master/proposed/cache.md
+ * @see https://www.php-fig.org/psr/psr-16/
  */
-interface CRM_Utils_Cache_Interface {
+interface CRM_Utils_Cache_Interface extends \Psr\SimpleCache\CacheInterface {
 
   /**
    * Set the value in the cache.
    *
    * @param string $key
    * @param mixed $value
+   * @param null|int|\DateInterval $ttl
+   * @return bool
    */
-  public function set($key, &$value);
+  public function set($key, $value, $ttl = NULL);
 
   /**
    * Get a value from the cache.
    *
    * @param string $key
+   * @param mixed $default
    * @return mixed
-   *   NULL if $key has not been previously set
+   *   The previously set value value, or $default (NULL).
    */
-  public function get($key);
+  public function get($key, $default = NULL);
 
   /**
    * Delete a value from the cache.
    *
    * @param string $key
+   * @return bool
    */
   public function delete($key);
 
   /**
    * Delete all values from the cache.
+   *
+   * NOTE: flush() and clear() should be aliases. flush() is specified by
+   * Civi's traditional interface, and clear() is specified by PSR-16.
+   *
+   * @return bool
+   * @see clear
+   * @deprecated
    */
   public function flush();
 
+  /**
+   * Delete all values from the cache.
+   *
+   * NOTE: flush() and clear() should be aliases. flush() is specified by
+   * Civi's traditional interface, and clear() is specified by PSR-16.
+   *
+   * @return bool
+   * @see flush
+   */
+  public function clear();
+
+  /**
+   * Determines whether an item is present in the cache.
+   *
+   * NOTE: It is recommended that has() is only to be used for cache warming type purposes
+   * and not to be used within your live applications operations for get/set, as this method
+   * is subject to a race condition where your has() will return true and immediately after,
+   * another script can remove it making the state of your app out of date.
+   *
+   * @param string $key The cache item key.
+   *
+   * @return bool
+   */
+  public function has($key);
+
 }
diff --git a/CRM/Utils/Cache/Memcache.php b/CRM/Utils/Cache/Memcache.php
index 630589e3d861..8e6e01dc51bb 100644
--- a/CRM/Utils/Cache/Memcache.php
+++ b/CRM/Utils/Cache/Memcache.php
@@ -31,6 +31,10 @@
  * @copyright CiviCRM LLC (c) 2004-2018
  */
 class CRM_Utils_Cache_Memcache implements CRM_Utils_Cache_Interface {
+
+  use CRM_Utils_Cache_NaiveMultipleTrait; // TODO Consider native implementation.
+  use CRM_Utils_Cache_NaiveHasTrait; // TODO Native implementation
+
   const DEFAULT_HOST = 'localhost';
   const DEFAULT_PORT = 11211;
   const DEFAULT_TIMEOUT = 3600;
@@ -109,10 +113,14 @@ public function __construct($config) {
   /**
    * @param $key
    * @param $value
+   * @param null|int|\DateInterval $ttl
    *
    * @return bool
    */
-  public function set($key, &$value) {
+  public function set($key, $value, $ttl = NULL) {
+    if ($ttl !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::set() should support non-NULL TTL");
+    }
     if (!$this->_cache->set($this->_prefix . $key, $value, FALSE, $this->_timeout)) {
       return FALSE;
     }
@@ -121,10 +129,14 @@ public function set($key, &$value) {
 
   /**
    * @param $key
+   * @param mixed $default
    *
    * @return mixed
    */
-  public function &get($key) {
+  public function get($key, $default = NULL) {
+    if ($default !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::get() only supports NULL default");
+    }
     $result = $this->_cache->get($this->_prefix . $key);
     return $result;
   }
@@ -132,18 +144,22 @@ public function &get($key) {
   /**
    * @param $key
    *
-   * @return mixed
+   * @return bool
    */
   public function delete($key) {
     return $this->_cache->delete($this->_prefix . $key);
   }
 
   /**
-   * @return mixed
+   * @return bool
    */
   public function flush() {
     // FIXME: Only delete items matching `$this->_prefix`.
     return $this->_cache->flush();
   }
 
+  public function clear() {
+    return $this->flush();
+  }
+
 }
diff --git a/CRM/Utils/Cache/Memcached.php b/CRM/Utils/Cache/Memcached.php
index 07315e53cebe..c1ad9cb75aaf 100644
--- a/CRM/Utils/Cache/Memcached.php
+++ b/CRM/Utils/Cache/Memcached.php
@@ -31,6 +31,10 @@
  * @copyright CiviCRM LLC (c) 2004-2018
  */
 class CRM_Utils_Cache_Memcached implements CRM_Utils_Cache_Interface {
+
+  use CRM_Utils_Cache_NaiveMultipleTrait; // TODO Consider native implementation.
+  use CRM_Utils_Cache_NaiveHasTrait; // TODO Native implementation
+
   const DEFAULT_HOST = 'localhost';
   const DEFAULT_PORT = 11211;
   const DEFAULT_TIMEOUT = 3600;
@@ -110,11 +114,15 @@ public function __construct($config) {
   /**
    * @param $key
    * @param $value
+   * @param null|int|\DateInterval $ttl
    *
    * @return bool
    * @throws Exception
    */
-  public function set($key, &$value) {
+  public function set($key, $value, $ttl = NULL) {
+    if ($ttl !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::set() should support non-NULL TTL");
+    }
     $key = $this->cleanKey($key);
     if (!$this->_cache->set($key, $value, $this->_timeout)) {
       CRM_Core_Error::debug('Result Code: ', $this->_cache->getResultMessage());
@@ -126,10 +134,14 @@ public function set($key, &$value) {
 
   /**
    * @param $key
+   * @param mixed $default
    *
    * @return mixed
    */
-  public function &get($key) {
+  public function get($key, $default = NULL) {
+    if ($default !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::get() only supports NULL default");
+    }
     $key = $this->cleanKey($key);
     $result = $this->_cache->get($key);
     return $result;
@@ -161,11 +173,15 @@ public function cleanKey($key) {
   }
 
   /**
-   * @return mixed
+   * @return bool
    */
   public function flush() {
     // FIXME: Only delete items matching `$this->_prefix`.
     return $this->_cache->flush();
   }
 
+  public function clear() {
+    return $this->flush();
+  }
+
 }
diff --git a/CRM/Utils/Cache/NaiveHasTrait.php b/CRM/Utils/Cache/NaiveHasTrait.php
new file mode 100644
index 000000000000..78ecc6788763
--- /dev/null
+++ b/CRM/Utils/Cache/NaiveHasTrait.php
@@ -0,0 +1,49 @@
+<?php
+/*
+ +--------------------------------------------------------------------+
+ | CiviCRM version 5                                                  |
+ +--------------------------------------------------------------------+
+ | Copyright CiviCRM LLC (c) 2004-2018                                |
+ +--------------------------------------------------------------------+
+ | This file is a part of CiviCRM.                                    |
+ |                                                                    |
+ | CiviCRM is free software; you can copy, modify, and distribute it  |
+ | under the terms of the GNU Affero General Public License           |
+ | Version 3, 19 November 2007 and the CiviCRM Licensing Exception.   |
+ |                                                                    |
+ | CiviCRM is distributed in the hope that it will be useful, but     |
+ | WITHOUT ANY WARRANTY; without even the implied warranty of         |
+ | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.               |
+ | See the GNU Affero General Public License for more details.        |
+ |                                                                    |
+ | You should have received a copy of the GNU Affero General Public   |
+ | License and the CiviCRM Licensing Exception along                  |
+ | with this program; if not, contact CiviCRM LLC                     |
+ | at info[AT]civicrm[DOT]org. If you have questions about the        |
+ | GNU Affero General Public License or the licensing of CiviCRM,     |
+ | see the CiviCRM license FAQ at http://civicrm.org/licensing        |
+ +--------------------------------------------------------------------+
+ */
+
+/**
+ *
+ * @package CRM
+ * @copyright CiviCRM LLC (c) 2004-2018
+ *
+ * The traditional CRM_Utils_Cache_Interface did not support has().
+ * To get drop-in compliance with PSR-16, we use a naive adapter.
+ *
+ * Ideally, these should be replaced with more performant/native versions.
+ */
+trait CRM_Utils_Cache_NaiveHasTrait {
+
+  public function has($key) {
+    // This is crazy-talk. If you've got an environment setup where you might
+    // be investigating this, fix your preferred cache driver by
+    // replacing `NaiveHasTrait` with a decent function.
+    $hasDefaultA = ($this->get($key, NULL) === NULL);
+    $hasDefaultB = ($this->get($key, 123) === 123);
+    return !($hasDefaultA && $hasDefaultB);
+  }
+
+}
diff --git a/CRM/Utils/Cache/NaiveMultipleTrait.php b/CRM/Utils/Cache/NaiveMultipleTrait.php
new file mode 100644
index 000000000000..b1f37a44908c
--- /dev/null
+++ b/CRM/Utils/Cache/NaiveMultipleTrait.php
@@ -0,0 +1,102 @@
+<?php
+/*
+ +--------------------------------------------------------------------+
+ | CiviCRM version 5                                                  |
+ +--------------------------------------------------------------------+
+ | Copyright CiviCRM LLC (c) 2004-2018                                |
+ +--------------------------------------------------------------------+
+ | This file is a part of CiviCRM.                                    |
+ |                                                                    |
+ | CiviCRM is free software; you can copy, modify, and distribute it  |
+ | under the terms of the GNU Affero General Public License           |
+ | Version 3, 19 November 2007 and the CiviCRM Licensing Exception.   |
+ |                                                                    |
+ | CiviCRM is distributed in the hope that it will be useful, but     |
+ | WITHOUT ANY WARRANTY; without even the implied warranty of         |
+ | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.               |
+ | See the GNU Affero General Public License for more details.        |
+ |                                                                    |
+ | You should have received a copy of the GNU Affero General Public   |
+ | License and the CiviCRM Licensing Exception along                  |
+ | with this program; if not, contact CiviCRM LLC                     |
+ | at info[AT]civicrm[DOT]org. If you have questions about the        |
+ | GNU Affero General Public License or the licensing of CiviCRM,     |
+ | see the CiviCRM license FAQ at http://civicrm.org/licensing        |
+ +--------------------------------------------------------------------+
+ */
+
+/**
+ *
+ * @package CRM
+ * @copyright CiviCRM LLC (c) 2004-2018
+ *
+ * The traditional CRM_Utils_Cache_Interface did not support multiple-key
+ * operations. To get drop-in compliance with PSR-16, we use a naive adapter.
+ * An operation like `getMultiple()` just calls `get()` multiple times.
+ *
+ * Ideally, these should be replaced with more performant/native versions.
+ */
+trait CRM_Utils_Cache_NaiveMultipleTrait {
+
+  /**
+   * Obtains multiple cache items by their unique keys.
+   *
+   * @param iterable $keys A list of keys that can obtained in a single operation.
+   * @param mixed $default Default value to return for keys that do not exist.
+   *
+   * @return iterable A list of key => value pairs. Cache keys that do not exist or are stale will have $default as value.
+   *
+   * @throws \Psr\SimpleCache\InvalidArgumentException
+   *   MUST be thrown if $keys is neither an array nor a Traversable,
+   *   or if any of the $keys are not a legal value.
+   */
+  public function getMultiple($keys, $default = NULL) {
+    $result = [];
+    foreach ($keys as $key) {
+      $result[$key] = $this->get($key, $default);
+    }
+    return $result;
+  }
+
+  /**
+   * Persists a set of key => value pairs in the cache, with an optional TTL.
+   *
+   * @param iterable $values A list of key => value pairs for a multiple-set operation.
+   * @param null|int|\DateInterval $ttl Optional. The TTL value of this item. If no value is sent and
+   *                                       the driver supports TTL then the library may set a default value
+   *                                       for it or let the driver take care of that.
+   *
+   * @return bool True on success and false on failure.
+   *
+   * @throws \Psr\SimpleCache\InvalidArgumentException
+   *   MUST be thrown if $values is neither an array nor a Traversable,
+   *   or if any of the $values are not a legal value.
+   */
+  public function setMultiple($values, $ttl = NULL) {
+    $result = TRUE;
+    foreach ($values as $key => $value) {
+      $result = $this->set($key, $value, $ttl) || $result;
+    }
+    return $result;
+  }
+
+  /**
+   * Deletes multiple cache items in a single operation.
+   *
+   * @param iterable $keys A list of string-based keys to be deleted.
+   *
+   * @return bool True if the items were successfully removed. False if there was an error.
+   *
+   * @throws \Psr\SimpleCache\InvalidArgumentException
+   *   MUST be thrown if $keys is neither an array nor a Traversable,
+   *   or if any of the $keys are not a legal value.
+   */
+  public function deleteMultiple($keys) {
+    $result = TRUE;
+    foreach ($keys as $key) {
+      $result = $this->delete($key) || $result;
+    }
+    return $result;
+  }
+
+}
diff --git a/CRM/Utils/Cache/NoCache.php b/CRM/Utils/Cache/NoCache.php
index 66c6b74cff41..3c1d9b944527 100644
--- a/CRM/Utils/Cache/NoCache.php
+++ b/CRM/Utils/Cache/NoCache.php
@@ -32,6 +32,9 @@
  */
 class CRM_Utils_Cache_NoCache implements CRM_Utils_Cache_Interface {
 
+  use CRM_Utils_Cache_NaiveMultipleTrait; // TODO Consider native implementation.
+  use CRM_Utils_Cache_NaiveHasTrait; // TODO Native implementation
+
   /**
    * We only need one instance of this object. So we use the singleton
    * pattern and cache the instance in this variable
@@ -54,20 +57,22 @@ public function __construct($config) {
   /**
    * @param string $key
    * @param mixed $value
+   * @param null|int|\DateInterval $ttl
    *
    * @return bool
    */
-  public function set($key, &$value) {
+  public function set($key, $value, $ttl = NULL) {
     return FALSE;
   }
 
   /**
    * @param string $key
+   * @param mixed $default
    *
    * @return null
    */
-  public function get($key) {
-    return NULL;
+  public function get($key, $default = NULL) {
+    return $default;
   }
 
   /**
@@ -86,4 +91,8 @@ public function flush() {
     return FALSE;
   }
 
+  public function clear() {
+    return $this->flush();
+  }
+
 }
diff --git a/CRM/Utils/Cache/Redis.php b/CRM/Utils/Cache/Redis.php
index 4988b8beef8d..01fc538c9355 100644
--- a/CRM/Utils/Cache/Redis.php
+++ b/CRM/Utils/Cache/Redis.php
@@ -33,6 +33,10 @@
  *
  */
 class CRM_Utils_Cache_Redis implements CRM_Utils_Cache_Interface {
+
+  use CRM_Utils_Cache_NaiveMultipleTrait; // TODO Consider native implementation.
+  use CRM_Utils_Cache_NaiveHasTrait; // TODO Native implementation
+
   const DEFAULT_HOST    = 'localhost';
   const DEFAULT_PORT    = 6379;
   const DEFAULT_TIMEOUT = 3600;
@@ -113,11 +117,15 @@ public function __construct($config) {
   /**
    * @param $key
    * @param $value
+   * @param null|int|\DateInterval $ttl
    *
    * @return bool
    * @throws Exception
    */
-  public function set($key, &$value) {
+  public function set($key, $value, $ttl = NULL) {
+    if ($ttl !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::set() should support non-NULL TTL");
+    }
     if (!$this->_cache->set($this->_prefix . $key, serialize($value), $this->_timeout)) {
       if (PHP_SAPI === 'cli' || (Civi\Core\Container::isContainerBooted() && CRM_Core_Permission::check('view debug output'))) {
         CRM_Core_Error::fatal("Redis set ($key) failed: " . $this->_cache->getLastError());
@@ -133,25 +141,27 @@ public function set($key, &$value) {
 
   /**
    * @param $key
+   * @param mixed $default
    *
    * @return mixed
    */
-  public function get($key) {
+  public function get($key, $default = NULL) {
     $result = $this->_cache->get($this->_prefix . $key);
-    return ($result === FALSE) ? NULL : unserialize($result);
+    return ($result === FALSE) ? $default : unserialize($result);
   }
 
   /**
    * @param $key
    *
-   * @return mixed
+   * @return bool
    */
   public function delete($key) {
-    return $this->_cache->delete($this->_prefix . $key);
+    $this->_cache->delete($this->_prefix . $key);
+    return TRUE;
   }
 
   /**
-   * @return mixed
+   * @return bool
    */
   public function flush() {
     // FIXME: Ideally, we'd map each prefix to a different 'hash' object in Redis,
@@ -159,7 +169,12 @@ public function flush() {
     // more general rethink of cache expiration/TTL.
 
     $keys = $this->_cache->keys($this->_prefix . '*');
-    return $this->_cache->del($keys);
+    $this->_cache->del($keys);
+    return TRUE;
+  }
+
+  public function clear() {
+    return $this->flush();
   }
 
 }
diff --git a/CRM/Utils/Cache/SerializeCache.php b/CRM/Utils/Cache/SerializeCache.php
index 6164694c3e5c..fdf0eb6d47b5 100644
--- a/CRM/Utils/Cache/SerializeCache.php
+++ b/CRM/Utils/Cache/SerializeCache.php
@@ -36,6 +36,9 @@
  */
 class CRM_Utils_Cache_SerializeCache implements CRM_Utils_Cache_Interface {
 
+  use CRM_Utils_Cache_NaiveMultipleTrait;
+  use CRM_Utils_Cache_NaiveHasTrait; // TODO Native implementation
+
   /**
    * The cache storage container, an array by default, stored in a file under templates
    */
@@ -67,10 +70,15 @@ public function fileName($key) {
 
   /**
    * @param string $key
+   * @param mixed $default
    *
    * @return mixed
    */
-  public function get($key) {
+  public function get($key, $default = NULL) {
+    if ($default !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::get() only supports NULL default");
+    }
+
     if (array_key_exists($key, $this->_cache)) {
       return $this->_cache[$key];
     }
@@ -85,32 +93,41 @@ public function get($key) {
   /**
    * @param string $key
    * @param mixed $value
+   * @param null|int|\DateInterval $ttl
+   * @return bool
    */
-  public function set($key, &$value) {
+  public function set($key, $value, $ttl = NULL) {
+    if ($ttl !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::set() should support non-NULL TTL");
+    }
     if (file_exists($this->fileName($key))) {
-      return;
+      return FALSE; // WTF, write-once cache?!
     }
     $this->_cache[$key] = $value;
-    file_put_contents($this->fileName($key), "<?php //" . serialize($value));
+    $bytes = file_put_contents($this->fileName($key), "<?php //" . serialize($value));
+    return ($bytes !== FALSE);
   }
 
   /**
    * @param string $key
+   * @return bool
    */
   public function delete($key) {
     if (file_exists($this->fileName($key))) {
       unlink($this->fileName($key));
     }
     unset($this->_cache[$key]);
+    return TRUE;
   }
 
   /**
    * @param null $key
+   * @return bool
    */
   public function flush($key = NULL) {
     $prefix = "CRM_";
     if (!$handle = opendir(CIVICRM_TEMPLATE_COMPILEDIR)) {
-      return; // die? Error?
+      return FALSE; // die? Error?
     }
     while (FALSE !== ($entry = readdir($handle))) {
       if (substr($entry, 0, 4) == $prefix) {
@@ -120,6 +137,11 @@ public function flush($key = NULL) {
     closedir($handle);
     unset($this->_cache);
     $this->_cache = array();
+    return TRUE;
+  }
+
+  public function clear() {
+    return $this->flush();
   }
 
 }
diff --git a/CRM/Utils/Cache/SqlGroup.php b/CRM/Utils/Cache/SqlGroup.php
index 0bb8e98e277a..3b983ae10632 100644
--- a/CRM/Utils/Cache/SqlGroup.php
+++ b/CRM/Utils/Cache/SqlGroup.php
@@ -38,6 +38,9 @@
  */
 class CRM_Utils_Cache_SqlGroup implements CRM_Utils_Cache_Interface {
 
+  use CRM_Utils_Cache_NaiveMultipleTrait; // TODO Consider native implementation.
+  use CRM_Utils_Cache_NaiveHasTrait; // TODO Native implementation
+
   /**
    * The host name of the memcached server.
    *
@@ -89,18 +92,28 @@ public function __construct($config) {
   /**
    * @param string $key
    * @param mixed $value
+   * @param null|int|\DateInterval $ttl
+   * @return bool
    */
-  public function set($key, &$value) {
+  public function set($key, $value, $ttl = NULL) {
+    if ($ttl !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::set() should support non-NULL TTL");
+    }
     CRM_Core_BAO_Cache::setItem($value, $this->group, $key, $this->componentID);
     $this->frontCache[$key] = $value;
+    return TRUE;
   }
 
   /**
    * @param string $key
+   * @param mixed $default
    *
    * @return mixed
    */
-  public function get($key) {
+  public function get($key, $default = NULL) {
+    if ($default !== NULL) {
+      throw new \RuntimeException("FIXME: " . __CLASS__ . "::get() only supports NULL default");
+    }
     if (!array_key_exists($key, $this->frontCache)) {
       $this->frontCache[$key] = CRM_Core_BAO_Cache::getItem($this->group, $key, $this->componentID);
     }
@@ -119,12 +132,14 @@ public function getFromFrontCache($key, $default = NULL) {
 
   /**
    * @param string $key
+   * @return bool
    */
   public function delete($key) {
     CRM_Core_BAO_Cache::deleteGroup($this->group, $key, FALSE);
     CRM_Core_BAO_Cache::$_cache = NULL; // FIXME: remove multitier cache
     CRM_Utils_Cache::singleton()->flush(); // FIXME: remove multitier cache
     unset($this->frontCache[$key]);
+    return TRUE;
   }
 
   public function flush() {
@@ -132,6 +147,11 @@ public function flush() {
     CRM_Core_BAO_Cache::$_cache = NULL; // FIXME: remove multitier cache
     CRM_Utils_Cache::singleton()->flush(); // FIXME: remove multitier cache
     $this->frontCache = array();
+    return TRUE;
+  }
+
+  public function clear() {
+    return $this->flush();
   }
 
   public function prefetch() {
diff --git a/Civi.php b/Civi.php
index ae5dc6f4b1dd..8acfdba76862 100644
--- a/Civi.php
+++ b/Civi.php
@@ -26,20 +26,15 @@ class Civi {
   public static $statics = array();
 
   /**
-   * EXPERIMENTAL. Retrieve a named cache instance.
-   *
-   * This interface is flagged as experimental due to political
-   * ambiguity in PHP community -- PHP-FIG has an open but
-   * somewhat controversial draft standard for caching. Based on
-   * the current draft, it's expected that this function could
-   * simultaneously support both CRM_Utils_Cache_Interface and
-   * PSR-6, but that depends on whether PSR-6 changes any more.
+   * Retrieve a named cache instance.
    *
    * @param string $name
    *   The name of the cache. The 'default' cache is biased toward
    *   high-performance caches (eg memcache/redis/apc) when
    *   available and falls back to single-request (static) caching.
    * @return CRM_Utils_Cache_Interface
+   *   NOTE: Beginning in CiviCRM v5.4, the cache instance complies with
+   *   PSR-16 (\Psr\SimpleCache\CacheInterface).
    */
   public static function cache($name = 'default') {
     return \Civi\Core\Container::singleton()->get('cache.' . $name);
diff --git a/composer.json b/composer.json
index 9d81135dbbec..1078ccf3d303 100644
--- a/composer.json
+++ b/composer.json
@@ -55,7 +55,8 @@
     "pear/Net_SMTP": "1.6.*",
     "pear/Net_socket": "1.0.*",
     "civicrm/civicrm-setup": "~0.2.0",
-    "guzzlehttp/guzzle": "^6.3"
+    "guzzlehttp/guzzle": "^6.3",
+    "psr/simple-cache": "~1.0.1"
   },
   "repositories": [
     {
diff --git a/composer.lock b/composer.lock
index 0cce99a2a89b..8f3e4d2659e9 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file",
         "This file is @generated automatically"
     ],
-    "content-hash": "9c5441f5ce4c51ed3a8cc326693cd904",
+    "content-hash": "233f9c457d9e7d49a6d96c356e1035f1",
     "packages": [
         {
             "name": "civicrm/civicrm-cxn-rpc",
@@ -1131,6 +1131,54 @@
             ],
             "time": "2012-12-21T11:40:51+00:00"
         },
+        {
+            "name": "psr/simple-cache",
+            "version": "1.0.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/php-fig/simple-cache.git",
+                "reference": "408d5eafb83c57f6365a3ca330ff23aa4a5fa39b"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/php-fig/simple-cache/zipball/408d5eafb83c57f6365a3ca330ff23aa4a5fa39b",
+                "reference": "408d5eafb83c57f6365a3ca330ff23aa4a5fa39b",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=5.3.0"
+            },
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "1.0.x-dev"
+                }
+            },
+            "autoload": {
+                "psr-4": {
+                    "Psr\\SimpleCache\\": "src/"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "PHP-FIG",
+                    "homepage": "http://www.php-fig.org/"
+                }
+            ],
+            "description": "Common interfaces for simple caching",
+            "keywords": [
+                "cache",
+                "caching",
+                "psr",
+                "psr-16",
+                "simple-cache"
+            ],
+            "time": "2017-10-23T01:57:42+00:00"
+        },
         {
             "name": "sabberworm/php-css-parser",
             "version": "6.0.1",