I found issue in serialize function. When I try to serialize object with unseted field I got the error:
[msgpack] (msgpack_serialize_zval) type is unsupported, encoded as null

Example code:

class MyClass
{
  protected $field;

  public function preSerialize()
  {
    unset($this->field);
  }
}

$t = new MyClass();
$t->preSerialize();
msgpack_serialize($t);

PHP Version 7.0.3
msgpack version 2.0.0 (header version 0.5.4)