Briefly, what is the difference between symmetric (single-key) and asymmetric (public-key or public/private-key) cryptography?



In Object Oriented programming, what is your favorite design pattern, and why?  Some example design patterns are singletons, factories, and facades.



The following regular expression is supposed to match an IP address.  What is wrong with it?  Can you improve it?
/[0-9]+.[0-9]+.[0-9]+.[0-9]+/



Can the following code's performance be increased at all, without changing the basic algorithm?  If so, how?  If not, why?  Can you tell me what this function calculates?
<?
function someFunction($n) {
	$x = $n;
	$i = 0;
	while (true) {
		$i = $i + 1;
		if ($i == 5)
			break;
		$x = $x + $n*(1/$x);
		$x = 1/2 * $x;
	}
	return $x;
}
?>