You can write a script that creates and returns a .gif, .jpeg or .png image using PHP for tracking purposes using the GD library (which is often distributed with PHP in modern versions). If you don't have access to GD, you can always recompile PHP with GD enabled.
Example:
pixel.php (commented for the purposes of explanation):
<?php
// Create an image, 1x1 pixel in size
$im=imagecreate(1,1);
// Set the background colour
$white=imagecolorallocate($im,255,255,255);
// Allocate the background colour
imagesetpixel($im,1,1,$white);
// Set the image type
header("content-type:image/jpg");
// Create a JPEG file from the image
imagejpeg($im);
// Free memory associated with the image
imagedestroy($im);
?>
In a simple example, you can then call this tracking pixel using the following example URL in an email or other page:
<img src="http://example.com/pixel.php?a=value1&b=value2&c=value3">
Using variables:
Within your pixel.php you can then parse and interpret any $_GET variables that are passed to it within the image tag, simplistically:
if (isset($_GET['a'])) {
// (Do|log) act on a
}
if (isset($_GET['b'])) {
// (Do|log) act on b
}
if (isset($_GET['c'])) {
// (Do|log) act on c
}
Apply and repeat as you need, but you can be quite sophisticated about what you do and especially as you have access to quite a lot of information about the user through being able to set vars on the $_GET string.
A more applicable example might be:
<img src="http://example.com/pixel.php?userid=98798&campaign=302&last=8">
Tracking more than just $_GET variables:
You can also pick up much more information using PHP, such as:
// Server variables
$ip = $_SERVER['REMOTE_ADDR'];
$referer = $_SERVER['HTTP_REFERER'];
$useragent = $_SERVER['HTTP_USER_AGENT'];
$browser = get_browser(null, true);
etc...
and then perhaps insert into a tracking table in your database:
$sql = "INSERT INTO campaign_tracking
('when','campaign','last','ip','useragent')
VALUES
(NOW(),'$campaign','$last','$ip','$useragent')";
This is a(the) basic method used widely for tracking email marketing campaigns and specifically in PHP, but the same method is applicable using other scripting/programming languages and libraries - and for other purposes too.
Further and useful information on GD:
