Redirect Page PHP

Hi, I have to do this redirect:

from (actual site with redirection)
https://mysite.com/folder1/folder2/page.php

to extermal site
https://folder1.mysite.com/folder2/page.php

I have tried with regex and without, but the result is always 404.

with regex

^/folder1/(.*)
https://folder1.mysite.com/$1

I did some testing and the problem seems to be “page.php”

what is the problem? where am i wrong?

Tnx
Regards