function user_data($user_id)
{
    $data          = array();
    $user_id       = (int) $user_id;
    $func_num_args = func_num_args();
    $func_get_args = func_get_args();

    if ($func_num_args > 1) {
        unset($func_get_args[0]);
        $fields = '`' . implode('`, `', $func_get_args) . '`';

        $data = mysql_fetch_assoc(mysql_query("SELECT $fields FROM `users` WHERE `user_id` = $user_id"));

        print_r($data);
        die();
        return $data;
    }        
}

  $data = mysql_fetch_assoc(mysql_query("SELECT $fields FROM `users` WHERE `user_id` = ". $user_id))

  $data = mysql_fetch_assoc(mysql_query("SELECT `". $fields ."` FROM `users` WHERE `user_id` = ". $user_id))

$res = mysql_query("SELECT $fields FROM `users` WHERE `user_id` = ". $user_id) or die(mysql_error());

$data = mysql_fetch_assoc($res);