How to debug php code when use www to communicate with server side.

I use www to send something to server side, and assign a php file as the receiver. But there might be something wrong in php code, because the data returned from server side is not correct. I want to debug into the php code to see what's wrong there. How can I accomplish that ? I have two php files, one is used to read the data from client, another is used to write data back to client.

I'm new to php, could anybody tell me how to debug php code in this case ? Thanks.