POO PHP - Sobreescritura de Métodos

<?php
class Valores
{
protected $valor1;
protected $valor2;
protected $resultado;

public function __construct($v1,$v2){
$this->valor1=$v1;
$this->valor2=$v2;
}

public function imprimir(){
return $this->resultado;
}
}

class Operar extends Valores
{
protected $title;

public function __construct($v1,$v2,$title)
{
parent::__construct($v1,$v2);
$this->title=$title;
}

public function Sumar(){
$this->resultado=$this->valor1+$this->valor2;
}

public function imprimir()
{
echo "<h3>".$this->title."</h3>";
echo "La suma de: ".$this->valor1." y ".$this->valor2. "es :";
echo parent::imprimir();
}
}

$obj=new Operar(5,1,'Sobreescritura de m&eacute;todos');
$obj->Sumar();
$obj->imprimir();
?>

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http://www.mediafire.com/download/h6gija3cwrs3a9r/Herencia_entre_clases_-02.zip