wrapping output of plugin function in admin interface

DESCRIPTION:
I am trying to write a plugin that adds a submenu to the Manage top-level menu. I want to put hyperlinks on my submenu page that link back to my plugin PHP file, which then grabs the query string I include in the URL and executes the appropriate function in my plugin PHP file.

PROBLEM:
This all works, but the output produced by the function I execute is NOT wrapped in the admin interface.

EXAMPLE:
here is a very barebones example of my code – basically, I run through some processing logic if my PHP receives a certain query string, otherwise, I add an action that results in my sub-menu being displayed.


if (isset($_GET['up_action'])) {
switch ($_GET['up_action']) {
case "checkout":
up_checkOut($up_filename);
break;
default:
die("Invalid action");
break;
}
} else {
add_action('admin_menu', 'uploadplus_add_admin_menu_pages');
}

function uploadplus_add_admin_menu_pages() {
add_management_page('UploadPlus', 'UploadPlus', 8, __FILE__, 'up_showForm');
}

function up_checkOut($filename) {
// THIS OUTPUTS HTML THAT DOESN'T INCLUDE THE ADMIN
// HEADER AND FOOTER - I WANT IT TO
print('<div class="wrap">'."n");
print('<a href="/wp-test/wp-content/files/'.$filename.'">Download file</a>');
print('</div>');
}

In my submenu page (code not shown), there is a hyperlink that I want to link back to this PHP page with a query string that will cause a the up_checkOut() function to run, which I want to do some stuff and then display the results. Here is an example of the link:


$queryStr='up_action=checkout&up_id='.$row['id'].
'&up_filename='.$row['filename'];
$url='/wordpress/wp-content/plugins/uploadplus/uploadplus.php?'.$queryStr;

print('<a href="'.$url.'">Checkout &amp; edit</a>');

The problem is, the up_checkOut() function displays the results correctly, but the HTML is not wrapped in the admin interface – i.e., the admin header and footer aren’t there.

I also tried using
/wp-admin/edit.php?page=uploadplus/uploadplus.php
as the page to link to with my additional query parameters
and it did include the admin header and footer, but gave me a bunch of error messages:

HOW DO I DO SOMETHING LIKE THIS? WHAT’S THE SUGGESTED WAY TO DO SOMETHING LIKE THIS? ARE THERE ANY EXAMPLES I CAN REFER TO?

Any help would be greatly appreciated!