php创建没有变量传递函数的类

Lets say I have a class with a function in it:

classes.php

 class Foobar{
        function writeWord($word){
               return $word;
        }
 }

On the page that i call this class i need to initialize the class, then call it later.

php2.php

require_once('classes.php');
$something = new Foobar();

 echo $something->writeWord('Dog');

The problem here is that php comes back and says Foobar is missing arguments on line 1. how can I solve this?