class Example
{
    private $value;

    public function __construct()
    {
        $this->value = "test";
    }

    public function __get($key)
    {
        if (property_exists($this,$key)) {
            return $this->{$key};
        } else {
            return null; // or throw an exception
        }
    }

    public function __set($key,$value)
    {
        return; // or throw an exception
    }
}

用法：

$example = new Example();
var_dump($example->value);
$example->value = "invalid";
var_dump($example->value);

输出：

string(4) "test"
string(4) "test"