当不在对象上下文中时使用$ this吗？

错误信息：

致命错误：不在第51行的class.db.php中的对象上下文中使用$ this

错误行：

            return $this->PDOInstance->prepare($sql, $driver_options);

代码：

class DB {   
        public $error = true; 

        private $PDOInstance = null;

        private static $instance = null;

        private function __construct()
          {
            try {

                $this->PDOInstance = new PDO('mysql:host='.HOST.';dbname='.DBNAME.';',
                                                    USER,
                                                    PASSWORD,
                                                    array(
                                                            PDO::ATTR_ERRMODE=>PDO::ERRMODE_EXCEPTION,
                                                            PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
                                                            ));

                $this->PDOInstance->query("SET NAMES 'cp1251'");
            } 
            catch(PDOException $e) { 
                echo "error";
                exit();
            }
          }


        public static function getInstance()
        {  
            if(is_null(self::$instance))
            {
              self::$instance = new DB();
            }
            return self::$instance;
        }


        private function __clone() {
        }

        private function __wakeup() {
        }         

        public static function prepare($sql, $driver_options=array())
        {
            try {
                return $this->PDOInstance->prepare($sql, $driver_options);  /// ERROR in this line
            } 
            catch(PDOException $e) { 
                $this->error($e->getMessage());
            }
        }

         }