mysql_query() problem

Hi, I'm getting the error <<Warning: Supplied argument is not a valid MySQL result resource>> from this code: function validateUser($username,$password) { $conn = connectToMySQL(); mysql_select_db($db_name,$conn); $query="select * from members where username='".$username."' AND password='".$password."'"; $result = mysql_query($query,$conn); if (mysql_num_rows($result) != 0) { //*this is the line that returns the error* return true; } else { return false; } } I've checked that the connection works, and that the query is what it should be, and the query works when I type it into the terminal. Any ideas? Thanks, Brian Rue