<?PHP
function replace_url ($content) {
	if (empty($content)) return;

	//给URL地址加上 <a> 链接
	$preg = '/(?:http:\/\/)?([\w.]+[\w\/]*\.[\w.]+[\w\/]*\??[\w=\&\+\%]*)/is';
	$content = preg_replace($preg,'<a href="http://\1" target="_blank">\1</a>',$content);

	return $content;
}

echo replace_url('百度:http://blog.csdn.net/moqiang02');
?>