PHP getter / setters

Fatal error: Call to undefined method Foo::stuff() in ...

我的代码：

class Foo extends FooBase{

  static $_instance;
  private $_stuff;

  public function getStuff($which = false){
    if($which) return self::app()->_stuff[$which]; else return self::app()->_stuff;
  }

  public function setStuff($stuff){
    self::app()->_stuff = $stuff;
  }

  public static function app(){
    if (!(self::$_instance instanceof self)){
      self::$_instance = new self();
    }

    return self::$_instance;
  }

}

Foo::app()->stuff = array('name' => 'Foo','content' => 'whatever');

echo Foo::app()->stuff('name'); // <- this doesn't work...

FooBase类看起来像这样：

class FooBase{

  public function __get($name){
    $getter = "get{$name}";
    if(method_exists($this,$getter)) return $this->$getter();
    throw new Exception("Property {$name} is not defined.");
  }

  public function __set($name,$value){
    $setter = "set{$name}";
    if(method_exists($this,$setter)) return $this->$setter($value);
    if(method_exists($this,"get{$name}"))
      throw new Exception("Property {$name} is read only.");
    else
      throw new Exception("Property {$name} is not defined.");

  }
}

所以,如果我理解正确,getter函数不能有参数？为什么？或者我在这里做错了什么？