A question about PHP object-oriented

I would like to ask you a question about PHP object-oriented. no matter whether the code design below is reasonable or not, we only need to look at the code execution results and why. & Lt ;? Phpclass & nbsp; aaa {& nbsp; function & nbsp; show () a question about PHP object-oriented
I would like to ask you whether the following code design is reasonable or not, but only the code execution, what the result will be, and why it is.

 Class aaa
{
Function show ()
{
Echo $ this-> str;
}
}

Class sss extends aaa
{
Protected $ str = '20140901 ';
}

$ S = new sss ();
$ S-> show ();
?>

------ Solution --------------------
Tell me where you don't understand
------ Solution --------------------
Subclass inherits the public method of the parent class, and overwrites the public method with the same name.

Do you understand this?