Singleton Pattern applied to a Service of Zend Framework

MemcacheRetriever.php

<?php

/**
 * This service return an instance of Zend Cache Manager Plugin.
 *
 * @author gdarrigo
 */
class Application_Service_MemcacheRetriever {

    protected static $_istance = null;
    
    protected $_cache;

    /**
     * Initialize Zend Cache Manager.
     */
    private function __construct() {
        $bootstrap = Zend_Controller_Front::getInstance()->getParam('bootstrap');

        if ($bootstrap->hasPluginResource('cachemanager')) {
            $cacheManager = $bootstrap->getPluginResource('cachemanager');

            $this->_cache = $cacheManager->getCacheManager()->getCache('memcached');
        }
    }
    
    /**
     * Prevent Singleton to be cloned.
     * 
     * @return boolean
     */
    private function __clone() {
        return new Exception('You can\'t clone a Singleton');
    }
    
    /**
     * Retrieve an istance of singleton.
     * 
     * @return type
     */
    public static function getIstance() {

        if (self::$_istance === null) {
            self::$_istance = new self();
        }

        return self::$_istance;
    }
    
    /**
     * Return Zend Cache Manager.
     * 
     * @return type
     */
    public function getCache() {
        return $this->_cache;
    }

}