我无法连接到我的数据库

This is my error:

MySQL ERROR: Access denied for user 'www-data'@'localhost' (using password: NO)

And this is my set.php

<?php
$sitename = "http://www.URL.COM";
$link = mysql_connect("(ip site)", "root", "password");
$db_selected = mysql_select_db('database', $link);
mysql_query("SET NAMES utf8");

function fetchinfo($rowname,$tablename,$finder,$findervalue) {
    if($finder == "1") $result = mysql_query("SELECT $rowname FROM $tablename");
    else $result = mysql_query("SELECT $rowname FROM $tablename WHERE `$finder`='$findervalue'");
    $row = mysql_fetch_assoc($result);
    return $row[$rowname];
}
?>

I search all time answer but with no success.

As you're not using password, why you write like this:

$link = mysql_connect("(ip site)", "root", "password");

If your username = "root" on localhost then try to write the connection like the following way:

<?php

$link = mysql_connect('localhost', 'root', '{Your Password}');
if (!$link) {
    die('Not connected : ' . mysql_error());
}

$db_selected = mysql_select_db('{Your DB Name}', $link);
if (!$db_selected) {
    die ('Can\'t use {Your DB Name} : ' . mysql_error());
}

For details check: MySQL

Note: Try to use PDO or mysqli

MySQLi

PDO