$output = bla blah bla; /* the data output from the background php page. I had to put SOMETHING here, */
$getdate = date( 'd-m-Y, H:i:s' );// Get the date.
$user_ip = $_SERVER['REMOTE_ADDR'];// Get the user IP, just in case.
$referer = getenv("HTTP_REFERER");// Get the refering page. just in case
$file = "./your path to logfile/logfile.csv"; /*define the file as required, csv opens easily in excell or imports to sql if REQ, but is really just a text file.*/
$fp = fopen($file, "a+"); //open the text file for writing.
// Write into the text file.
fputs ($fp, "$getdate, $user_ip, $referer, $output\n"); //see note after $output definition above
fclose($fp);// Close the text file.