PHP : Set document root to be a subdirectory without .htaccess or Virtualhost

Recently I faced a problem with hosting a website on a server to which I have limited access. I was provided with permission only to a sub-directory on the server.
I'm habituated to use relative paths, but my root points to the root of server and not my sub-directory. This forced me to use absolute paths at every point.
I searched for the solution and end up with two results:

.htaccess ot Virtualhost

I never tried to understand .htaccess and I don't have access to set Virtualhost. This forced me to try bit harder and here is the outcome.

I've created a file named rootpath.php on root directory of my website.
(Note: this is not root '/' of the web server, but my sub-directory)
Its contents are as follows:

<?php
    $relative_path = "/subdirectory";
    $server_url = $_SERVER["SERVER_NAME"];

    $root = "http://" . $server_url. $relative_path;
?>

Now, I include this file on top of every file.

Eg: <?php require('rootpath.php'); ?>

and append all paths with <?php print $root; ?>

Eg: <a href="<?php print $root; ?>/index.php">

Note: Initially I used './' and '../' notation, but it won't work always. Consider the following case, which I faced:

/index.php
/header.php
/footer.php
/scripts.php
/contact
--contact/contact.php

I defined a common header for all the pages and stored it in header.php, which in-turn imports all the necessary scripts from scripts.php.
Now, if you import header.php from contact/contact.php, it work work properly because scripts.php is loaded relatively in header.php and now the control searches it in contact directory.