Hello,
I used to create a prototype of remote thermal printer using Raspberry Pi 3 and USB thermal printer. The printer connected to the server using WiFi connection. I create a python application for the printer, so it listen for incoming message from server. When the printer received the messages, it will print the receipt using serial connection.
In your case, the printer has gprs module built-in. So i think, the only missing parts are:
1. The printer server application (on your server)
2. Integrate your PHP application to the server.
I can help you setup and create the application using Python2 or PHP7. Please let me know if you're interested.
Regards