Where should I put this file?

[eluser]Pert[/eluser]
If you must use just PHP you can put it anywhere you want, project root next to index.php or you can create your own subfolder.

I would suggest you try to convert it to a controller if at all feasible, it will give you all the nice CodeIgniter features.

As far as I can see this file doesn't really do that much so I can see the logic in not asking it to load CodeIgniter etc.

Are you using URL rewrite?