Apache 2.4

Default format:

https://httpd.apache.org/docs/2.4/logs.html#errorlog

https://httpd.apache.org/docs/2.4/mod/core.html#errorlogformat

This plugin supports only PHP error logs. At the moment, there are no plans to add support for other log formats.

OK.
When error_log is undefined PHP errors end up in the webserver’s log.

BTW I usually simulated that in PHP error log:
https://github.com/szepeviktor/wordpress-fail2ban/blob/fd1bf4911d9dee50d635a30266688234fdaad0c9/mu-plugin/wp-fail2ban-mu-instant.php#L240-L260