The concept of relative is always from the file being executed.
ralph l mayo explains with an example here
Note that the relative starting point is always from the file being executed. Ie., if you have:
if index.php (the file the users requests) includes ‘../include.php’, include.php must include otherinclude as ‘otherinclude.php’ and not ‘myapp/otherinclude.php’
For this reason if you have includes spanning several directories or includes that are used in different contexts, it’s often easier to define a root path and include absolutely.