Just the a quick knock-up to add monolog to a class. You could just instantiate the logger in the function you're debugging, but this makes it easier to bounce around the class.

gistfile1.php

<?php

use Monolog\Logger;
use Monolog\Handler\StreamHandler;

class foo extends bar
{
    private $log = null;

    public function __construct($swiftmailer, $service_container)
    {
        // if your class extends, make sure you check for the parents constructor and pass whatever you need
        parent::__construct($swiftmailer, $service_container);
        $this->log = new Logger('name');
        $this->log->pushHandler(new StreamHandler('/path/to/app/logs/audit.log', Logger::WARNING));
    }
    
    public function whateverNeedsDebugging()
    {
        $this->log->addWarning('Foo');
        $this->log->addError('Bar');
    }   
}