[imp] how to move a custom_login.php out of ./horde/imp/scripts?

[Alain Fauconnet]

Hello readers,

I need to implement a custom login page. I have managed 
to make ./horde/imp/scripts/custom_login.php work successfully
as follows:

$server_key = '<the proper server key>';

$url = '';
instead of '/'

This way, I can log in OK if I open
http://<domain>/horde/imp/scripts/custom_login.php
from my browser.

So far so good (any explanation for the needed changes
besides the server key?)

Now I want to move this script at DocumentRoot
and rename it to index.php (and later put our
contents in of course).
I've changed:
define('IMP_BASE', '<the absolute path of IMP base>');
instead of just '..'.
After this this, if I open http://<domain>/index.php I get the login
form, but logging in doesn't work anymore. I get
redirected to the standard login page.

So...
- can this script be moved out of the Horde directory tree?
- can it be renamed? (guess so!)
- what should I change to make it work while at DocumentRoot?

horde-3.0.5, imp-h3-4.0.3

Thanks for any tip.
Greets,
_Alain_