[Boofo]

This will also work for IPv4:

// Checks for valid IP Address since some proxies will send invalid user
// agent strings.
//
// usage: if (!isValidIPv4($ipaddress))

function isValidIPv4($addr) {
	$octs = explode('.',$addr);
	if (count($octs) != 4) return false;

	foreach ($octs as $oct) {
		if (!ctype_digit($oct)) return false;
		$oct = intval($oct);
		if ($oct < 0 || $oct > 255) return false;
	}
	return true;
}