相对路径怎样转化为绝对路径

[php]代码库

//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;
}