PHP有多个子类，可以逐个访问子类

我不完全确定这是理解问题还是语法问题。我正在尝试从同一父级下的其他子类访问子类。

    class TopLevel {
    foreach (glob("assets/php/*.php") as $filename) // will get all .php files within plugin directory
      {
        include $filename;
        $temp = explode('/',$filename);
        $class_name = str_replace('.php','',$temp[count($temp)-1]); // get the class name 
        $this->$class_name = new $class_name;   // initiating all plugins
      }
    }
    class A extends TopLevel {
        $var = 'something';
        public function output() {
            return $this->var;
        }
    }
    class B extends TopLevel {
        // This is where I need help, CAN the child class A be accessed sideways from class B? Obviously they need to be loaded in correct order of dependency.
        $this->A->output();
    }

我不明白为什么这不起作用。不是很好的结构，但它是一个单对象应用程序。