如何找出PHP类不支持下划线变量的原因？

I have quite a weird question. I'm currently new to OOP in PHP, so please do not blame me for this question.

I have created a file and a class, called: Animals.

class Animals {

   protected $_snake;

}

I have also created another file with another class, called: Nature. I would wanted to use the variable defined in the Animals class so I did extend this class. (Is it the right method to use another class's variables?)

<?php

include 'Animals.php';

class Nature extends Animals {

   public function __construct () {

      echo $this -> _snake;

   }
}

?>

I have used the underscore to mark, that it is a protected variable (is it correct?). The problem occurs when I try to run this code, becuse I always get this error:

Notice: Undefined property: Nature::$_snake in C:\***\Nature.php on line ***

However if I remove the underscore before the variable name, it works perfectly.

Can anyone please explain what did I do wrong when applied that underscore to the variable?

EDIT:

When I insert the:

protected $_snake;

to the Nature class, it works well with the underscore! I'm quite confused now.