Log errors can be really useful when your website is online to not show errors to the client or to simply log when you develop a theme or plugin.

Parvula suggest Seldaek/Monolog to log what you want.

Use monolog

1. Firstly install Monolog composer require monolog/monolog
2. If you want to log error and exception, enable the log logErrors: true in system.yaml
3. It is recommended to disable the debug because Whoops block the current exception and throw a new exception. debug: true in system.yaml

To log what you want, use the service registered as ‘logger’ in the router ($this->logger).

A basic example could be:

$this->logger->addError('Hello world');