There are no generators of a code. You self should create classes of messages. 

The detailed description http://memo.undr.su/2009/10/15/realizaciya-google-protocol-buffers-na-php/ (Inside Russian)

Example of usage of library:

protobufer file:

message Message_Example {
    required int32 id = 1;
    required sint32 balance = 2;
    required bool isAdmin = 3;
    enum Status {
        active = 0;
        inactive = 1;
        deleted = 2;
    }
    required enum Status status = 4;
    required string name = 5;
    required bytes bytes = 6;
}

php code:

<?php
/**
 * Class of message
 *
 */
class Message_Example extends PhpBuf_Message_Abstract {
    public function __construct() {
        $this->setField("id", PhpBuf_Type::INT, PhpBuf_Rule::REQUIRED, 1);
        $this->setField("balance", PhpBuf_Type::SINT, PhpBuf_Rule::REQUIRED, 2);
        $this->setField("isAdmin", PhpBuf_Type::BOOL, PhpBuf_Rule::REQUIRED, 3);
        $this->setField("status", PhpBuf_Type::ENUM, PhpBuf_Rule::REQUIRED, 4, array("active", "inactive", "deleted"));
        $this->setField("name", PhpBuf_Type::STRING, PhpBuf_Rule::REQUIRED, 5);
        $this->setField("bytes", PhpBuf_Type::BYTES, PhpBuf_Rule::REQUIRED, 6);
    }
}

/**
 * Work with message
 */
require_once("/phpbuf/lib/PhpBuf.php");

$message = new Message_Example();
$writer = new PhpBuf_IO_Writer();
$message->id = 150;
$message->balance = -12345;
$message->isAdmin = true;
$message->status = "deleted";
$message->name = "Andrey Lepeshkin";
$message->bytes = "Some bytes";
$message->write($writer);
                
$reader = PhpBuf_IO_Reader::createFromWriter($writer);
$message = new Message_Example();
$message->read($reader);
                
$this->assertEquals(150, $message->id);
$this->assertEquals(-12345, $message->balance);         
$this->assertTrue($message->isAdmin);
$this->assertEquals("deleted", $message->status);               
$this->assertEquals("Andrey Lepeshkin", $message->name);                
$this->assertEquals("Some bytes", $message->bytes);  

?>

RPC call:
----

RPC call required implements: http://code.google.com/p/protobuf-socket-rpc/

package com.example.protobuf;

message Request {
    message Hoge {
        required string value = 1;
    }
}
message Response {
    message Foo {
        repeated string values = 1;
    }
}

service MyService {
    rpc sayHello(Request.Hoge) returns(Response.Foo);
}

php code:

<?php

class Request_Hoge extends PhpBuf_Message_Abstract {
    public function __construct(){
        $this->setField('value', PhpBuf_Type::STRING, PhpBuf_Rule::REQUIRED, 1);
    }
    public static function name(){
        return __CLASS__;
    }
}
class Response_Foo extends PhpBuf_Message_Abstract {
    public function __construct(){
        $this->setField('values', PhpBuf_Type::STRING, PhpBuf_Rule::REPEATED, 1);
    }
    public static function name(){
        return __CLASS__;
    }
}

class MyService extends PhpBuf_RPC_Service_Client {
    public function __construct(PhpBuf_RPC_Context $context){
        parent::__construct($context);
        $this->setServiceFullQualifiedName('com.example.protobuf.MyService');
        $this->registerMethodResponderClass('sayHello', Response_Foo::name());
    }
}

$request = new Request_Hoge;
$request->value = 'hello world';

$ctx = new PhpBuf_RPC_Context;
$ctx->addServer('127.0.0.1', 12345);
$ctx->addServer('127.0.0.1', 67890);

$service = new MyService($ctx);
$response = $service->sayHello($request);

?>