php如何将相对路径转换为绝对路径

http://enenba.com/中有这么个链接：tool/201215.html
怎样使之转换为http://enenba.com/tool/201215.html

百度知道一个问题

给你一个函数：

<?php
/*
$content：网页内容；
$feed_url：网站域名；
*/
function relative_to_absolute($content, $feed_url) { 
	preg_match('/(http|https|ftp):\/\//', $feed_url, $protocol); 
	$server_url = preg_replace("/(http|https|ftp|news):\/\//", "", $feed_url); 
	$server_url = preg_replace("/\/.*/", "", $server_url); 
	if ($server_url == '') { 
		return $content; 
	} 
	if (isset($protocol[0])) { 
		$new_content = preg_replace('/href="\//', 'href="'.$protocol[0].$server_url.'/', $content); 
		$new_content = preg_replace('/src="\//', 'src="'.$protocol[0].$server_url.'/', $new_content); 
	} else { 
		$new_content = $content; 
	} 
	return $new_content; 
} 
?>

end