Hi Daryl, now I can send the message to the chat room. I don't know why, I just go to Admin Console and change Topic Description for chat room, and then I used this code to send the message & it was sent.
This is my code I used to send the message to chat room. I hope this will help someone face with this problem like me.
$conn = new XMPPHP_XMPP('my_server_name', 5222, 'my_account', 'my_password', 'xmpphp', 'my_server_name', $printlog = true, $loglevel = XMPPHP_Log::LEVEL_INFO);
$conn -> connect();
$conn -> processUntil('session_start');
$conn -> presence(NULL, "available", "room01@conference.my_server_name/my_account");
$conn -> message('room01@conference.my_server_name', 'Test message!', 'groupchat');
sleep(60);
$conn -> presence(NULL, "unavailable", "room01@conference.my_server_name/my_account");
$conn -> disconnect();
Thank you so much Daryl, you helped me a lot!!!