problema mysql_fetch_array()

[Electro]

Ciao a tutti,
non riesco a capire perchè ma php eseguendo questa operazione:

  $db = sqlconnectstr($host_bb, $database_bb, $user_bb, $password_bb);
  $query = "SELECT userid,usergroupid,username,password FROM user WHERE username='$_POST[username]'";
  $result = mysql_query($query, $db);
  $row = mysql_fetch_array($result);
  $password = md5($_POST['password']); 

mi da questo errore: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in xxxxxxxxxxxxxxxxxxxxxxxxxx on line 22.

Ciao e grazie in anticipo