PHP relative paths

suppose that I am working on a similar server PHP, with more projects, each of them with multiple subfolders and includes. Let’s concentrate on proj1:

/var/www/html
    /proj1
        /inc
            header.php
       /assets
            styles.css
            script.js
        page1.php
        index.php
   /proj2
    ...

Now, I want to include the css and the js with a relative path (not absolute) from header.php, but if I use $_SERVER["DOCUMENT_ROOT"] it returns to /var/www/html.
What do you think it could be the better solution to echo "script.js" in such scenario, without relying on absolute paths?
Thanks

PS: For the sake of clarity, we work on nested directories and structures, so I am searching for a way in which the relative path can change on these different folders, so it needs to be dynamic.