从子文件夹站点包含HTML

I have two sites, one stored in the root of server, another in a sub-folder as shown below.

SITE ONE
ROOT/index.php
ROOT/assets/includes/header.php - contains styles and scripts

SITE TWO
ROOT/SUB-FOLDER/index.php
ROOT/SUB-FOLDER/assets/includes/header.php - contains styles and scripts

I am using PHP includes in order to include external files where the first site works fine using the code below, however the second one don't:

include($_SERVER["DOCUMENT_ROOT"] . '/assets/includes/header.php');

Could you please help me to achieve similar result for the site stored in the SUB-FOLDER.

Thanks in advance.

Changing the include link to:

include('assets/includes/header.php'); 

and adding the

<base href="http://example.com/SUB-FOLDER">

tag to head of the included pages solved the my issue.