<p>How can I unset variable? For example, PHP has an <code>unset($var)</code> function.</p>

You can't. There's no notion of "unsetting" a variable. You can set it to a different value - 0, null, whatever's appropriate. Instance/static variables don't even have a concept of whether the variable is set/unset, and local variables only have "definitely assigned" or "not definitely assigned".
What is it you're trying to achieve?