You can set a repeat count and an interval for sending Gratuitous ARP packet by using set command of pid_ioctl function. PHPoC sends 4 Gratuitous arp packets with an interval of 2 seconds when it boots. If you want to manually send the packet, you can use this command.
pid_ioctl($pid, "set garp RC INTV");
RC means a repeat count and INTV means an interval.
This example shows how to set Gratuitous arp.
<?php $pid = pid_open("/mmap/net0"); // open NET 0 (Ethernet) pid_ioctl($pid, "set garp 5 10"); // set GARP (5 times at an interval of 10 secs) ?>