set user group using ZKLibrary

Question

i need help to create a function in zkteco php library to change user group this is my code but its not working,anybody can help me

reference url : https://github.com/adrobinoga/zk-protocol/blob/master/sections/access.md zklibrary : https://github.com/kamshory/ZKLibrary

`public function userGroup($uid,$gid)
{
    $command = CMD_USERGRP_WRQ;
    $byte1 = chr((int) ($uid % 256));
    $byte2 = chr((int) ($uid >> 8));
    $byte3 = chr((int) ($gid));
    $command_string = $byte1 . $byte2 . $byte3;
    return $this->execCommand($command, $command_string);
}`

this function is not ok i think the $command_string is wrong but i cant rectify the error