uso di mysql_real_escape_string?

[neorf]

Ho una connessione al db mysql che funziona correttamente con:

$posted_name = $_POST['post_nome'];
$posted_pass = $_POST['post_password'];

$db_conn = mysql_connect($db_host,$db_user,$db_password);
if($db_conn== FALSE) die("Errore connessione al db");
mysql_select_db($db_name,$db_conn) or die("Err conn db"); 

Poi provo a passare il login come:

$post_nome = mysql_real_escape_string($_POST['post_nome']);
$post_password = mysql_real_escape_string($_POST['post_password']); 

e mi scappa questo errore:

Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: Access denied for user 'www-data'@'localhost' (using password: NO) in check_session.php on line 10

Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: A link to the server could not be established in check_session.php on line 10

Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: Access denied for user 'www-data'@'localhost' (using password: NO) in check_session.php on line 11

Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: A link to the server could not be established in check_session.php on line 11 

Come mai?

Thanks

neorf