You might post your PHP code that works to see what you have. Below shows how to append a GET request (query_string, the string after the ?) to a URL. In windows, apache sets the query_string as an environmental varable, from which an application started by apache collects the data. Note that some characters have special meaning and can't be put in a query_string.
client.println("GET /arduino.txt HTTP/1.0");
With query_string added:
client.println("GET /arduino.php?put-your-data-here HTTP/1.0");