php-modbus/examples/example_fc4.php

<?php

use PHPModbus\ModbusMasterUdp;

// Create Modbus object
$modbus = new ModbusMasterUdp("192.192.15.51");

try {
	// Read input discretes - FC 4
	$recData = $modbus->readMultipleInputRegisters(0, 0, 2);
} catch (Exception $e) {
	// Print error information if any
	echo $modbus;
	echo $e;
	exit;
}

var_dump($recData);