为什么我的函数不起作用 PHP

嘿，为什么我的函数在这里不起作用是php代码，它是用mysql_connect写的：

function isUserLoggedIn() {
    global $conn;
    $sql = "SELECT user_id, password FROM user
            WHERE
            user_id='" . fixstr($loggedInUser->user_id) . "' 
            AND 
            password='" . fixstr($loggedInUser->password) . "' 
            AND
            active = 1
            LIMIT 1";
    $res = mysql_query($sql);
    $rs = mysql_fetch_array($res);  
    if($loggedInUser($res) == NULL)
    {
        return false;
    }
    else
    {
        //Query the database to ensure they haven't been removed or possibly banned?
        if(returns_result($sql) > 0)
        {
                return true;
        }
        else
        {
            //No result returned kill the user session, user banned or deleted
            $loggedInUser->userLogOut();
            return false;
        }
    }
}

连接确实返回一个活动窗口，但无法连接到任何函数 有人知道为什么我的代码不起作用吗？