php-modbus/examples/example_fc6.php

<?php
require __DIR__ . '/../vendor/autoload.php';

use PHPModbus\ModbusMasterUdp;

require_once __DIR__ . '/request_input_data.php'; // 'ip', 'unitid','reference','quantity' are read from $_GET
$value = isset($_GET['value']) ? ((int)$_GET['value']) : -1000;

$modbus = new ModbusMasterUdp($ip);

try {
    // FC6
    $recData = $modbus->writeSingleRegister($unitId, $reference, [$value]);
} catch (Exception $e) {
    // Print error information if any
    echo $modbus;
    echo $e;
    exit;
}

echo '<h1>Status</h1><pre>';
print_r($modbus);
echo '</pre>';
echo '<h1>Data</h1><pre>';
print_r($recData);
echo '</pre>';