[Solution] Couldn't find constant Monolog\Logger::DEBUG
Please keep in mind that this post was written more than 2 years ago and might be outdated.
Note for others having same problem
PHP Warning: constant(): Couldn't find constant Monolog\Logger::DEBUG in [..] /vendor/symfony/monolog-bundle/Symfony/Bundle/MonologBundle/DependencyInjection/MonologExtension.php on line 110
This is because composer version you are running doesn\'t support PSR-4. to fix it update your composer with