Imitation of a non-existent site - is this possible?

Question

PHP

Task
Login to the site admin panel is carried out at:
admin.sitename.ru?fpvmoeurhv985hg34h93g84h3948hg3g
Everything that is indicated after the question mark is the key to "activate the admin panel".
Question
So the question is, is it possible to issue a message about the "non-existent" site, if the wrong key is specified or it is not specified at all?
Explanation
Why? All in order to completely hide the admin panel, thereby making life more peaceful :)
(PHP + Apache)
PS 403, 404 codes and redirects are not suitable.

Answer 1

What do you have in mind?
To make it look like the host doesn't exist?
No. The browser has already resolved admin.sitename.ru to the ip address and connected to it on port 80.
Maybe the usual "redirect" supposedly from all subdomains will suffice (provided that you have configured a redirect from subdomains)?

Answer 2

Kindergarten, pants with straps. "Let's get under the covers one more time so that the ghosts won't find us."
Instead of normal security, we invent "secrets with glass".
No.

Answer 3

You seem to understand that if this happens via http, then such a link is easy to sniff and follow it? )
By subject - show the same thing that is shown if you go to the server by ip. It won't raise suspicion. But it's all bullshit. Tighten authorization through htaccess.