In HTML, I can find a file starting from the web server's root folder by beginning the filepath with "/". Like:

/images/some_image.jpg

I can put that path in any file in any subdirectory, and it will point to the right image.

With PHP, I tried something similar:

include("/includes/header.php");

...but that doesn't work.

I think that that this page is saying that I can set include_path once and after that, it will be assumed. But I don't quite get the syntax. Both examples start with a period, and it says:

Using a . in the include path allows for relative includes as it means the current directory.

Relative includes are exactly what I don't want.

How do I make sure that all my includes point to the root/includes folder? (Bonus: what if I want to place that folder outside the public directory?)

Clarification

My development files are currently being served by XAMPP/Apache. Does that affect the absolute path? (I'm not sure yet what the production server will be.)