class a {

    public $attr2 = 'a';
    private $final_attr = '1';

    function __construct() {

        $vars = get_class_vars(get_class($this));
        foreach($vars as $k => $val) {
            if(substr($k,0,strlen('final_')) == 'final_') continue;
            $v = 'final_'.$k;
            if(isset($this ->$v )) {
                echo 'class : ' . get_class($this) . " Can't declare parent's attribute!";
                exit();
            }
        }
    }

    function __set($attr,$val) {

        $v = 'final_'.$attr;
        if(isset($this -> $v)) {
            $this -> $v = $val;
        }
    }

    function __get($k) {

        $v = 'final_'.$k;
        if(isset($this -> $v)) {
            return $this -> $v;
        }
    }
}

class b extends a {

    public $attr = 'b';

    function __construct() {
        parent::__construct();
    }
}

$a = new b();