php如何将输出写入xml

我有这个代码：

I have this code:

$rss = '<?xml version="1.0" encoding="ISO-8859-1" ?>';
		$rss .= '<rss version="2.0">';
		$rss .= '<channel>';
		$rss .= '<title>Lavasoft</title>';
		$rss .= '<link>/blog</link>';
		$rss .= '<description>Malware Encyclopedia</description>';
		
		$result = mysql_query('SELECT title, teaser, timestamp FROM blog', $db);
		while($row = mysql_fetch_array($result)) {
			$rss .= '<item>';
			$rss .= '<title>'.$row['title'].'</title>';
			$rss .= '<link>'.'/blog/'.$row['title'].'</link>';
			$rss .= '<description>'.$row['teaser'].'</description>';
			$rss .= '<pubDate>'.$row['timestamp'].'</pubDate>';
			$rss .= '</item>';
		}
		
		$rss .= '</channel>';
		$rss .= '</rss>';

如果有新的更新，如何自动写入xml？

How to auto write to xml when there's new update?