Address.php

<?php

class Address {
	public $bairro;
	public $logradouro;
	public $uf;

	public function get_address($cep)
	{
		$url = "http://viacep.com.br/ws/".$cep."/xml";
		$xml = simplexml_load_file($url);
		$this->bairro = $xml->bairro;
		$this->logradouro = $xml->logradouro;
		$this->uf = $xml->uf;
	}

}

index.php

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

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

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