php替换字符串中所有url为链接

把字符串中的url替换成可点击的链接，默认给链接添加nofollow，即不允许搜索引擎搜索

function replace_urls($string, $rel = 'nofollow'){
    $host = "([a-z\d][-a-z\d]*[a-z\d]\.)+[a-z][-a-z\d]*[a-z]";
    $port = "(:\d{1,})?";
    $path = "(\/[^?<>\#\"\s]+)?";
    $query = "(\?[^<>\#\"\s]+)?";
    return preg_replace("#((ht|f)tps?:\/\/{$host}{$port}{$path}{$query})#i", "<a href=\"$1\" rel=\"{$rel}\">$1</a>", $string);
}