php-fpm child process exited on signal 11

While googling for this same issue, and trying hard to find a solution that was not related to sessions (because I have ruled that out) nor to bad PHP code (because I have several websites running precisely the same version of WordPress, and none have issues... except for one), I came upon an answer telling that a possible solution did involve removing some buggy extension (usually memcache/d, but could be something else).

Since I had this same site working flawlessly on one Ubuntu server, when switching to a newer server, I immediately suspected that it was the migration from PHP 5.5 to 7 that caused the problem. It was just strange because no other website was affected. Then I remembered that another thing was different on this new server: I had also installed New Relic. This is both an extension and a small server that runs in the background and sends a lot of analytics data to New Relic for processing. Allegedly, it's a PHP 5 extension, but, surprisingly, it loads well on PHP 7, too.

Now here comes the tricky bit. At some point, I had installed W3 Total Cache for the WordPress installation of that particular website. Subsequently, I saw that the performance of that server was so stellar that W3TC was unnecessary, and simply stuck to a much simpler configuration. So I could uninstall W3TC. That's all very nice, but... I forgot that I had turned New Relic on W3TC, too (allegedly, it adds some extra analytics data to be sent to New Relic). When uninstalling W3TC, probably there was 'something' left on the New Relic configuration in my server which was still attempting to send data through the W3TC interface (assuming that W3TC has an interface... I really have no idea how it works at that level), and, because that specific bit of code was missing, the php_fpm handler for that website would fail... some of the time. Not all the time, because I'm assuming that, in most cases, nginx was sending static pages back. Or maybe php_fpm, set to 'recycle' after 100 calls or so, would crash-on-stop. Whatever exactly was happening, it was definitely related to New Relic — as soon as I removed the New Relic extension from PHP, that website went back to working normally.

Because this is such a specific scenario, I'm just writing this as an answer, in the remote chance that someone in the future googles for the exact problem.

In my case it was related to zend debug/xdebug. It forwards some TCP packets to the IDE (PhpStorm), that was not listening on this port (debugging was off). The solution is to either disable these extensions or enable debug listening on the debugging port.

It can happen if PHP is unable to write the session information to a file. By default it is /var/lib/php/session. You can change it by using configuration session_save_path.

phpMyAdmin having problems on nginx and php-fpm on RHEL 6