There is no logging system built-in, but only a service that loads an instance of the Monolog\Logger class. In order to use this service run the following composer command:
composer require monolog/monolog
Without any configuration, the default logger writes log messages
to the file logs/main.log file.
If you want to alter the default logger, you can specify this in
a Log.php configuration file.
The logger from the above configuration file writes to the default log file,
but also to php://stderr. Let's test it:
You may run this example:
php -S localhost:8000 -t examples/logging
If you visit e.g. http://localhost:8000
You will get a couple of log message in logs/main.log,
and because we use the log instance from config/Log.php class
the same messages are written to php://stderr.
For more information visit Monolog\Logger.