$_SESSION['valid_user_name'] = getUserId($username, $password);


function getUserId($username, $password)
{
    $username = addslashes($username);
    $username = mysql_real_escape_string($username);
    $password = addslashes($password);
    $password = mysql_real_escape_string($password);

    $conn = db_connect();

    $result = $conn->query("select id from user where username='$username' and password= sha1('$password')");
    if (!$result) {
        throw new Exception('Could not retrieve your user id.');
    }
    if ($result->num_rows > 0) {
        return $result;
    } else {
        throw new Exception('Could not retrieve your user id.');
    }
}