PHP中的单例表现异常

Question

我正在尝试在PHP中实现单例，但出于某种原因，每次静态变量“$ instances”为null并且它都会被重新初始化。

  class Enteliprint_Model_EnteliService {

        public static $instances;

        static public function getSingltoneInstances() {                  DebugBreak(); 
            if (NULL == Enteliprint_Model_EnteliService::$instances) {
                $result = array();          
                foreach (Enteliprint_Model_EnteliPrint::getEnteliPrintInstances(True) as $instance) {   
                    $associatedSceddules = json_decode($instance['AssocSchedules']);
                    $scheduleIDs = '-1,';                    
                    foreach ($associatedSceddules as $key=>$schedule) {
                        $scheduleIDs = $scheduleIDs.($key > 0 ? ', ' : '').$schedule->ID;
                    }                  
                    $instance['AssocFilters'] = Enteliprint_Model_EnteliFilter::getEnteliFilternstances($scheduleIDs);
                    $result[] = $instance;
                }
                Enteliprint_Model_EnteliService::$instances = $result;
            }
            return Enteliprint_Model_EnteliService::$instances;
        }

        public static function sendEvents($events) {                                  
            $instances = Enteliprint_Model_EnteliService::getSingltoneInstances(); 

有什么想法吗？