Using file_get_contents within admin but keep logged in

Quick question as I'm not having much luck finding an answer on Google.

I'm running a file_get_contents in a plugin I'm building to get the HTML of another page within the admin. The HTML is showing but the problem is, it's loading the /wp-login.php page HTML and not the specified URL.

$data = file_get_contents('wp-admin/options-general.php?page=another_page');
$html_sourcecode_get = htmlentities($data);
echo $html_sourcecode_get;

Does anyone know how to keep the HTML grab as if I was still logged in? Do I need to use cURL with credentials?

Note: I have tried the wp file system alternative and it returns a critical error.

Thanks.