I'm looking for the best approach to send data(String) from my server (PHP) to one specific android device. I can check the device if is online/offline on my server(with http-post method). But I don't know how to send data to spesific device without GCM. (I'm not looking for push notification and I'll send message to maximum 100 user.)
Without using GCM, you have several options:
From all of those I would look into polling first. It's often considere somewhat a bad practice, but it's not necessarily the case. Have a look at this AT&T blog post for some measurements.