php 相对路径转化成绝对路径

//相对路径转化成绝对路径 
<? 
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;  
}  
?>