<?php

//content of config.php
define('FRONTROOT', dirname(__FILE__).'/');
define('BACKROOT', dirname(__FILE__).'/admin/');

?>

<?php

//content of /home/username/public_html/index.php aka frontend
require(dirname(__FILE__).'/config.php');

print_r(FRONTROOT); # should print /home/username/public_html/
print_r(BACKROOT); # should print /home/username/public_html/admin/

?>

<?php

//content of /home/username/public_html/admin/index.php aka backend
require(dirname(__FILE__).'/../config.php');

print_r(FRONTROOT); # should print /home/username/public_html/
print_r(BACKROOT); # should print /home/username/public_html/admin/

?>