This project deals with an implementation of the basic functionality of the Modbus TCP and UDP based protocol using PHP. It's a copy of the releases from the project page over at Google Code with composer support added.

• Modbus master
  • FC1 - Read coils
  • FC2 - Read input discretes
  • FC3 - Read holding registers
  • FC4 - Read holding input registers
  • FC5 - Write single coil
  • FC6 - Write single register
  • FC15 - Write multiple coils
  • FC16 - Write multiple registers
  • FC22 - Mask Write register
  • FC23 - Read/Write multiple registers

 // Modbus master UDP
 $modbus = new ModbusMaster("192.168.1.1", "UDP");
 // Read multiple registers
 try {
     $recData = $modbus->readMultipleRegisters(0, 12288, 5);
 }
 catch (Exception $e) {
     // Print error information if any
     echo $modbus;
     echo $e;
     exit;
 }
 // Print data in string format
 echo PhpType::bytes2string($recData);

For more see documentation or FAQ.