Address.php

<?php

class Address {

	public function get_address($cep)
	{
		$url = "http://viacep.com.br/ws/".$cep."/xml";
		return simplexml_load_file($url);
	}

}

index.php

<?php
require('classes/Address.php');

$address = new Address();
$cep = "36204-220";
$newAddress = $address->get_address($cep);

echo "CEP Informado: ".$cep."<br />";
echo "Rua:".$newAddress->logradouro."<br />";
echo "Bairro:".$newAddress->bairro."<br />";
echo "Estado:".$newAddress->uf."<br />";