How can I try debugging the "Unknown: Headers already sent. You cannot change the session module's ini settings at this time in Unknown on line 0" warning, when PHP is not giving any more details except that it happened (no info on what's trying to send the headers or where the output had started).
This warning happens from time to time only. (In my case, it happens when I browse a Woocommerce store. It has happened in the local dev environment when I click on a product, when I click on adding product to the cart, when I click to checkout. But it happens rarely. Mostly, all the requests are processed all right, no PHP warnings.)
Almost everything that I found about "headers already sent" on the Internet, refers to the cases when the PHP code is written in a way that outputs some content before sending the headers, in which case the PHP gives a reference to the PHP code that's in some way related to the error. But the "unknown" cases are different.
I found that this error may be caused by repeatedly loading the same PHP extensions, or by some PHP extension working incorrectly. For example, "zlib" extension has had such problem when executing "ob_gzhandler".
I looked at the PHP configuration and didn't see an extension being loaded more than once. Regarding "ob_gzhandler", I found only one place in the Wordpress code using it, and added a log there. Then I got a headers already sent warning message, but the "ob_gzhandler" had not been used in that request.
So, how could this warning be tracked down? Can I use XDebug somehow to do it?
I'm using PHP 7.4 FPM.
