Firstly, while you are developing your code, use this at the top of each PHP page, this will return the errors thrown so you can easily identify where your code errors occur -
<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);
?>
Looking at your code, you have a no return data because of some operators missing at this line -
header("Location:https://brandstuckers.com/index.html?mobi=" .$mobi);
header("Location:https://brandstuckers.com/index.html?mobi=" .$mobi. "");
If this does not help, let us know on which line the error occured and what error were returned.
Looking at your edited question, you are calling a re-direct to index.html, your code is however in index.php
I will then suggest that you rather store all the variables in a session and call it from there. You can do this easily enough, read up on sessions from
THIS PHP manual.
In your login.php, right at the top as the first line of code, add the following -
if (session_status() == PHP_SESSION_NONE) {
session_start();
session_regenerate_id(true);
error_reporting(E_ALL);
ini_set('display_errors', 1);
}
$_SESSION['mobi'] = $mobi;
In your index.php page (maybe look at naming this page different as index pages is your landing or 1st page in your site, then followed by login or whichever other page) -
<input type="hidden" id="nn" style="display-inline;float:right;padding-right:1%" name="mobil" value="<?php echo $_SESSION['mobi']; ?>"/>
You can add as many variables to your sessions as you wish, just read up more on closing sessions, flushing variables and arrays etc.