When a POST request is received, the session is reset. Why does this happen and how to treat it?

Question

PHP

Hello. I created a test.php file on the site, I wrote three lines in it:

<?php
session_start();
print session_id();

So if from the outside (not from the site where this file is located, but from any other site or just from a file on my desktop) send a POST request to this file, then the session is redefined. Those. A new session is started and the old one is overwritten. Does anyone know what this is and how to treat it?