With the recent passage of PSR-3, a standard for a common interface for logging libraries, it seems appropriate to discuss the implementation with my favorite logging library. log4php is an open source Apache project that is a very versatile logging framework. Through log4php's configuration files, logging has a variety of output locations, so it's possible to send specific log levels to different output locations.
He includes the Composer requirements for the interface and shares the code for a wrapper class that implements the Logger interface and defines methods for each of the logging levels (alert, notice, debug, etc). Also in the post is an example XML configuration for log4php and how to load it into your class instance.