What is the best way to pass a string from PHP to another over different domain?

Question

I have one Apache server running on my raspberry pi and another on a PC. I have an RFID reader connected to raspberry pi which is continually collecting data from nearby tags. I want to pass this single string from raspberry pi to the PC over the server. I looked up into different methods such as CRUL, Ajax, Streams. I have developed python server with sockets, and a basic file_get_contents. I want to do it using $_GET method with Ajax and pass data as Json data format. After using many different combinations I'm getting more and more confused about this server communications. I assume the easiest way would be to use json_encode and json_decode, but even this gives me NULL value. Can someone direct me to right starting point, or give an example of how to do this using $_GET method on both ends?