<?php
    class OSses {
        private $names = array();

        public function __set($name, $value) {
            if (!in_array($name, array('windows', 'dos')))
                $this->names[$name] = $value;
        }

        public function getNames() {
            return $this->names;
        }
    }

    $osses = new OSses();
    $osses->linux = 'rocks';
    $osses->windows = 'blows';

    var_dump($osses->getNames());
? >

array(1) {
  ["linux"]=>
  string(5) "rocks"
}