Login virker ikke

Login virker ikke, selvom jeg skriver den rigtige koder og det rigtige brugernavn, håber at der er en som kan hjælpe:
Dette er min form:
<form action="login/checkuser.php" method="post" width="100">
<p>Username:<br><input type="text" SIZE=15,12; name="brugernavn"><br>
<p>Password:<br><input type="password" SIZE=15,12; name="kodeord"><br>
<input type="submit" value="Login" name="ok">
</form>

Dette er checkuser.php
<?
session_start(); 

include 'db.php';

$username = $_POST['username'];
$password = $_POST['password'];

if((!$username) || (!$password)){
?><script>self.location='../index.php?page=tryagain';</script><?
return;
}


$password = md5($password);


$sql = mysql_query("SELECT * FROM users WHERE username='$username' AND password='$password' AND activated='1'");
$login_check = mysql_num_rows($sql);

if($login_check == 1) {
    while($row = mysql_fetch_array($sql)){
    foreach( $row AS $key => $val ){
        $$key = stripslashes( $val );
    }
       
        session_register('first_name');
        $_SESSION['first_name'] = $first_name;
        session_register('last_name');
        $_SESSION['last_name'] = $last_name;
        session_register('email_address');
        $_SESSION['email_address'] = $email_address;
        session_register('user_level');
        $_SESSION['user_level'] = $user_level;
        session_register('username');
        $_SESSION['username'] = $username;
       
        mysql_query("UPDATE users SET last_login=now() WHERE userid='$userid'");
       
        header("Location: login_success.php");
    }
} else {
    echo "You could not be logged in! Either the username and password do not match or you have not validated your membership!<br />
    Please try again!<br />";
    include 'login_form.html';
}
?>¨

håber at en som kan hjælpe