Exemplo de classe PHP

<?php
    class Classe{
        private $a;

        public function __set($atributo, $valor) {
            switch($atributo) {
                case "a":
                    $this->$a = $valor;
                break;
            }
        }

        public function __get($atributo) {
            switch($atributo) {
                case "a":
                    return $this->$a;
                break;
            }
        }

        public function Decodifica() {
            return base64_decode($this->a);
        }
    }

    $classe = new Classe();
    $classe->a = "IkV1IHF1ZSBu428gZnVtbyBxdWVyaWEgdW0gY2lnYXJybw0KRXUgcXVlIG7jbyBhbW8gdm9j6g0KRW52ZWxoZWNpIGRleiBhbm9zIG91IG1haXMgbmVzc2Ug+mx0aW1vIG3qcyI=";
    echo $classe->Decodifica();
?>