I been learning php and ajax from w3schools but I have come across a simply question for which I cant find an answer to.
To request something from a php file I use a xmlhttpRequest object and specify the url (of that php file). Does this mean one php file for one request only? Let's say on a webpage there are a user log-in box and a comment box, I would need two php files to take the requests? I always thought the server side will have one main file that handle all the requests, each request from client will have a ID to specify what the request is and send back the necessary data to client. So what is the right?
I read a lot of material online, but everything is just basic example with just one request and one response.
You can use the same file for multiple requests. You can supply parameters along with the AJAX request, either by including them in the URL after ? (they'll be available in $_GET and $_REQUEST) or by using the POST method and sending them as form data (they'll be available in $_POST and $_REQUEST). You can use the Javascript FormData API to encode this properly; see the documentation here. Using the jQuery library can simplify all of this.
One of the parameters can then be a command or operation code, and the script can take different actions based on this.