The eight severity levels supported by PSR-3 are based on RFC 5424, and are described below:
Emergency – the system is unusable
Alert – immediate action is required
Critical – critical conditions
Error – errors that do not require immediate attention but should be monitored
Warning – unusual or undesirable occurrences that are not errors
Notice – normal but significant events
Info – interesting events
Debug – detailed information for debugging purposes
References:
http://www.sitepoint.com/logging-with-psr-3-to-improve-reusability/
https://tools.ietf.org/html/rfc5424 (page 10)