Lately I have seen more and more libraries picking up PSR-3 when it comes to logging. What a lot of libaries do wrong is that they depend on a concrete implementation of PSR-3, e.g. Mongolog instead of relying on the PSR-3 interface. From what I have seen this is because loggers get instantiated directly within the class. This is not a bad thing but it couples your code to a concrete implementation of PSR-3 which in turn means that there's no interoperability.