Save a string in an own webpage

Is there a way to save a string into a webpage ( with server on the internet ).

What I mean is that :

1- An android application sends a string to a website (No problem here ), so is there a way to let the website to listen for any connection from the android application (by the website's IP and the server port) ?

2- If not, then I can let the android sends the string as a filling textbox on the webpage (also no problem here) , so then, is there a way to save this string on the server ?

3- So at end, It become easy to get the saved string by any program or application by reading/ fetching the webpage ..

Colloquially, I don't know how webpages are programmed and how things go into it !! and I need help to put me on the right way to start studying how to achieve what I need.

To simplify the question : I need to save a string on a webpage so that, the webpage becomes a mediator to transfer data between sender and receiver. As I can do this by WiFi (local network) and I wanna do it globally (through Internet) ..