Problems with user auth

[Vedran]

Hi,
I'm trying to integrate your Simple User Auth script, and I had a lot of bugs and problems, but there is only one that I can't really figure out...
When I try to log in using script, I get following error

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /***/***/usermenu.php on line 28

Apperantly there is a problem with

PHP Code:

iif(mysql_num_rows($query) > 0) {
            
                    setcookie("loggedin",true,time()+170000);
                    $_SESSION['loggedin']=true; 


What is the problem and can u help me fix it?