Creating a 1×1 transparent GIF response in PHP

transparent.php

<?php
//saves ip address and timestamp
$str=date("Y-m-d H:i:s") . ": ". $_SERVER['REMOTE_ADDR'] . "n";
file_put_contents("ip_list.txt", $str, FILE_APPEND);

Sourced from: 

header("content-type: image/gif");
//43byte 1x1 transparent pixel gif
echo base64_decode("R0lGODlhAQABAIAAAAAAAAAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==");
?>