Display post in pop up page

Hi,

I didn’t find any answer for my problem browsing google 🙁
I want to display a single post (last of its gategory) in a pop-up page. Not a lightbox, a pop up.
The problem is all WP function (get header, have post…) returns fatal error when called in my pop up page. I put it in root of my theme (with other pages).

Any ideas ?
Thanks.

Edit : made some tests. I can open a pop up and use WP functions when opening page.php (pages model). But it seems I can’t use a custom page i created by myself…