snmpwalk under NT 4.0

Hi all, forgive me if this has been covered before.
I am trying to perform a simple snmpwalk such as:

<?php snmpwalkt("123.123.123.123", "community", "system.SysContact.0"); ?>

I always get the same response:

Fatal error: Call to undefined function: snmpget() in d:\.......\snmp.php on
line 1

I think it has something to do with the php.ini file, but not sure where to
go. Any information would be much appreciated.