Apache server falls off with regular POST requests, how to fix it?

Question

PHP

I am doing one project on the stock exchange theme. There it is required to update stock quotes every 15 seconds (AJAX). Everything works fine, but after 1-2 hours an incomprehensible error pops up:

PHP Notice: Undefined variable: _POST in ...

At the same time, all AJAX requests fall off, or rather they pass, but now the POST variable is not passed to them (see error). The same error is present in any other application running in the current environment (for example, the same phpmyadmin). In this case, GET requests are processed.
After a while, Apache itself also crashes, after a reboot everything becomes normal.
There is a suspicion that there is some kind of buffer overflow, but small data is transmitted there in each request, so it's strange.
Who has any ideas what could be the matter and how to find out the exact cause?

Answer 1

first you need to look in the system log, where the reason for the fall will most likely be written