public static function hash($input, $timestamp, $iteration, $salt) {
		$iteration = intval($iteration);
		if ($iteration <= 0) {
			return false;
		}

		for ($i = 0; $i < $iteration; ++$i) {
			$input = base_convert(sha1($input.$salt.$timestamp), 16, 36);
		}
		return $input;
	}