when parse a message, got an error field

[violetdeng]

I'm struggling with the bytes or utf-8 problem, the value of nick attribute is "nihao_1", while to array it changes to "1", I can't find out the reason
PHP version is 5.4
here's my codes

message server_user_account_t{
required string source_number = 1;
required bytes nick = 2;
required uint32 server_id = 3;
optional uint32 sex = 4;
}

$body = server_user_account_t::parseFromString($str);
var_dump($body);

the print message is

object(hy\protobuf\server_user_account_t)#25 (4) {
    ["source_number":protected]=>
    string(9) "757559966"
    ["nick":protected]=>
    string(7) "nihao_1"
    ["server_id":protected]=>
    string(1) "1"
    ["sex":protected]=>
    NULL
  }

$body = $body->toArray();
var_dump($body);

the print message is

array(3)` {
  ["source_number"]=>
  string(9) "757559966"
  ["nick"]=>
  int(0)
  ["server_id"]=>
  int(1)
}