在PHP中覆盖派生类中的静态成员

<?PHP
class Base {
  protected static $c = 'base';

  public static function getC() {
    return self::$c;
  }
}

class Derived extends Base {
  protected static $c = 'derived';
}

echo Base::getC(); // output "base"
echo Derived::getC();    // output "base",but I need "derived" here!
?>

那么最好的解决方法是什么？