How to use include_once in php with subdomains

Sidebar note:  You may also want to know about this function:
http://php.net/manual/en/function.getcwd.php

And if you want to write your code in a way that allows for cross-platform functionality, consider using the context-aware predefined constant: DIRECTORY_SEPARATOR

I don't know about the soon-to-be-extinct list.  My use of PHP goes back to 1999, so I encountered PHP on Windows 98, 2000, XP, ME, etc.  I have encountered the '/' problem in the past, when building a church web site circa 2000, and that was when I started using the constant.  Never looked back.

I can't remember the exact timeline.  I changed to DIRECTORY_SEPARATOR long ago because the '/' did not work as desired on Windows hosting.   I can't really say when Microsoft made a change because DIRECTORY_SEPARATOR has worked perfectly from day one, and I never had any reason to change back.  IIRC, this was in the days of PHP4, but it's been a long time.

This page has some copyright information that seems to point to 2004, since PHP 4.0.6.  So I guess Microsoft changed behaviors some time after that.
https://pear.php.net/reference/PHP_Compat-latest/PHP_Compat/_PHP_Compat-1.6.0a3---Compat---Constant---DIRECTORY_SEPARATOR.php.html