The Slim application’s environment will always contain a key slim.errors with a value that is a writable resource to which log and error messages may be written. The Slim application’s log object will write log messages to slim.errors whenever an Exception is caught or the log object is manually invoked.

If you want to redirect error output to a different location, you can define your own writable resource by modifying the Slim application’s environment settings. I recommend you use middleware to update the environment:

class CustomErrorMiddleware extends \Slim\Middleware
    public function call()
        // Set new error output
        $env = $this->app->environment;
        $env['slim.errors'] = fopen('/path/to/output', 'w');

        // Call next middleware

Remember, slim.errors does not have to point to a file; it can point to any valid writable resource.