The following works perfectly:

// Call this function with the following parameters:
// $1 = port number (eg 5900 for VNC)
// $2 = IP address (xxx.xxx.xxx.xxx)


transport = CreateSNMPTransport($node);
if (transport == null)
           return -1;
oid = ".1.3.6.1.2.1.6.13.1.4." . $2 . "." . $1 ; // IP connections
vars = SNMPWalk(transport, oid);
if (vars == null)
           return -2; // SNMPWalk failed
count = 0;
foreach (v: vars) {
  //trace(1, "SNMP WALK ".v->name."=".v->value);
  count = count +1;
}
return count;