The severities are (according to RFC3164) the PHP constants:
LOG_EMERG # Emergency: system is unusable
LOG_ALERT # Alert: action must be taken immediately
LOG_CRIT # Critical: critical conditions
LOG_ERR # Error: error conditions
LOG_WARNING # Warning: warning conditions
LOG_NOTICE # Notice: normal but significant condition
LOG_INFO # Informational: informational messages
LOG_DEBUG # Debug: debug-level messages