Add PHP Predefined Constants

[m-fedosov]

Previously, there was no need for predefined constants. The developers knew on which OS KPHP projects were launched. Now we're in open-source. Someone runs programs under Windows, and someone on Linux. Because of this, there is a need for predefined constants that change depending on the running system.

What constants need to be added:
https://www.php.net/manual/en/dir.constants.php

Why are they useful?

Well, since we still have a discussion here. Then here's an argument for why constants can be omitted.