服务器一变量多个用户可以同时修改，怎么实现

class TShare {
  private static $_Instance;
  private $memcache;
  private function __construct() {
    $this->memcache = new Memcache;
    $this->memcache->connect('127.0.0.1', 11211) or die ("Could not connect");
  }
  public static function getInstance() {
    if(empty(self::$_Instance)) self::$_Instance = new self();
    return self::$_Instance;
  }
  function __set($name, $val) {
    $this->memcache->set($name, $val, false, 1000);
  }
  function __get($name) {
    return $this->memcache->get($name);
  }
}