Sometimes extensions are compiled into PHP rather than loaded as separate modules. Before fiddling with the PHP.ini files, you should probably run a quick PHP script to see what modules are loaded:
<?php
var_dump(get_loaded_extensions());
You should also beware of PHP directives like disable_functions which may be disabling the pcntl functions. On my Ubuntu workstation, the pcntl functions are disabled with this directive:
/etc/php5/apache2/php.ini:disable_functions = pcntl_alarm,pcntl_fork,pcntl_waitpid,pcntl_wait,pcntl_wifexited,pcntl_wifstopped,pcntl_wifsignaled,pcntl_wexitstatus,pcntl_wtermsig,pcntl_wstopsig,pcntl_signal,pcntl_signal_dispatch,pcntl_get_last_error,pcntl_strerror,pcntl_sigprocmask,pcntl_sigwaitinfo,pcntl_sigtimedwait,pcntl_exec,pcntl_getpriority,pcntl_setpriority
I suggest you try a broader grep search to see if pcntl gets mentioned in other places:
grep -ir pcntl.so /etc/php/7.2
I don't have a /etc/php/7.2/cli/conf.d/20-pcntl.ini file
That seems a bit weird to me. You might try searching for more php.ini files there:
sudo find /etc -name "php.ini"
You should also check to make sure that you aren't running a different PHP from the command line than you are with apache. Under certain circumstances (which regrettably happen more often that you might think) your CLI PHP is different than your Apache PHP. This article discusses that issue in more detail.
If I disable this extension on Apache's .ini, it won't load on Apache but loads on Cli.
That actually sounds reasonable to me. If I had to guess, I would say that the wise Ubuntu/Debian package manager folks decided it would be unwise to allow pcntl_x functions to run in an apache environment, especially given this stark and ancient warning. I don't recall if that warning is outdated now or not. I would refer you to this other thread where I asked some related questions and was told I should use PHP-FPM with apache in event mode.
All that said, I have had some tremendous success writing "multithreaded" applications in PHP (technically not multithreaded but rather multiprocessing) using pcntl_fork and posix_setsid and the like.
To summarize, I'm guessing that PCNTL is already loaded both on apache and cli and the wise Ubuntu devs probably disabled the pcntl functions in the php.ini.
