OK - thank you for the explanation.
What I have is this (a some more code, with other functionality).
char url = "http://www.mywebserver.dk/arduino/arduino_update_data.php?";
status = client.connect(df_server, 80);
sprintf(buffer, "CHN0=%d&CHN1=%d&CHN2=%d&CHN3=%d", chn0_t, chn1_t, chn2_t, chn3_t);
client_close_cnt = 30; //close after 30 x 100 ms = 3 sec
nLCDUpdate(NO_LCD_CMD, 19, 0, TXT_DATA_INDICATOR_ERR, NULL);
4 analogue channels are read every 5 sec. Values are transmitted to webserver.
The "arduino_update_data.php" script grabs the data and saves them in a .csv file.
Asynch to this, when a user opens the webpage, a second .php script generates a graph, reads data from .csv files and shows them in the graph. Buttons for clearing data etc. are also generated and shown.
Everything works like a charm :-)
Now what I would like to have, was a button where I could trigger functionality in the Arduino, like Start / Stop the logging etc.
Any inputs on how this could be done ?