The SNMP extension provides a very simple and easily usable toolset for managing remote devices via the Simple Network Managment Protocol.
As it is a wrapper around the underlying Net-SNMP (or UCD-SNMP on older systems) library, all basic concepts are the same and the PHP functions change their behaviour depending on the Net-SNMP configuration files and environment variables.
More information about Net-SNMP can be found on http://www.net-snmp.org/.