Hi @jazibzaman
Did you try the global variable solution I gave you before?
Also, could you please check if your data.php can get extra parameters from the $_GET variable like the following?
In template:
<li><a href="<?php echo get_stylesheet_directory_uri(); ?>/includes/tabs/data.php?postid=123" class="tab">HTML</a></li>
In data.php:
$group_ID = $_GET['postid'];
This page should give you more idea about it: http://php.net/manual/en/reserved.variables.get.php.
Hope this helps!