evercookie_png.php

<?php
/* evercookie, by samy kamkar, 09/20/2010
 *  http://samy.pl : code@samy.pl
 *
 * This is the server-side variable PNG generator for evercookie.
 * If an HTTP cookie is passed, the cookie data gets converted into
 * RGB-values in a PNG image. The PNG image is printed out with a
 * 20-year cache expiration date.
 *
 * If for any reason this file is accessed again WITHOUT the cookie,
 * as in the user deleted their cookie, the code returns back with
 * a forced "Not Modified" meaning the browser should look at its
 * cache for the image.
 *
 * The client-side code then places the cached image in a canvas and
 * reads it in pixel by pixel, converting the PNG back into a cookie.
 *
 * -samy kamkar
 */

// we don't have a cookie, user probably deleted it, force cache
if (!$_COOKIE["evercookie_png"])
{
	header("HTTP/1.1 304 Not Modified");
	exit;
}

// width of 200 means 600 bytes (3 RGB bytes per pixel)
$x = 200;
$y = 1;

$gd = imagecreatetruecolor($x, $y);

$data_arr = str_split($_COOKIE["evercookie_png"]);

$x = 0;
$y = 0;
for ($i = 0; $i < count($data_arr); $i += 3)
{
	$color = imagecolorallocate($gd, ord($data_arr[$i]), ord($data_arr[$i+1]), ord($data_arr[$i+2]));
	imagesetpixel($gd, $x++, $y, $color);
}
 
header('Content-Type: image/png');
header('Last-Modified: Wed, 30 Jun 2010 21:36:48 GMT');
header('Expires: Tue, 31 Dec 2030 23:30:45 GMT');
header('Cache-Control: private, max-age=630720000');

// boom. headshot.
imagepng($gd);

?>