CLIENT CODE:
function Login()
{
    form f = {username, password};
    WWW request = SendRequest("https:/.../login.php", f);

    wait request;

    if(request.error != NULL || request.message == "ERROR")
        return "Error";
    else
        {
            if(request.message == "LOGIN_OK")
                GoForwardAndLoadNextThing();
        }
}


SERVER CODE:
function ProcessLogin(form f)
{
    Query q = SELECT username WHERE user = f.user AND password = f.password;

    QueryResult result;
    q.DoQuery(result);

    // If there was a match
    if(result != 0)
        die("LOGIN_OK")
    else
        die("ERROR");
}